Yes your Rule 3. is incomplete..
There is some description here
that bachur/levita identified(or came up himself!) with 5 rules but for rule 3, there are ceratin exceptions which i've listed here
- first letter of a word.
- second of two shvas under consecutive letters.
- after a tenua gedolah(long vowel), where the long vowel has no primary stress.(also some other cases, and can even be after a short vowel where the short vowel is "lengthened"/treated as a long vowel!)
- under a dagesh
- if a shva appears under the first of two consecutive identical letters (e.g., the first lamed of halleluyah)
re rule 3- also, and reversing the effect of any stress of vowel shift from a nasog achor or suffix on the noun/verb. (The rule is often misstated as just after a long vowel)
(rule 3 is weird because it means it can even be after a short vowel)
a simpler form of rule 3 would be long vowel with no primary stress.. but it is more complex than that.. as noted above..
Other cases to know about besides those 5 rules, are vav patach prefix, and vav shuruk prefix. I mention details re those below..
And another thing to know about is meteg can play a role according to some traditions, maybe moreso sephardi tradition.
And worth bearing in mind that in ashkenazi tradition i'm not sure if there is a long/short kamatz, given that in ashkenazi tradition the kamatz mark has one pronunciation.
In the case of a vav shuruk prefix, I don't know whether it's a long vowel or a short vowel. But either way, if there's a shva following it, then in ashkenazi tradition, it's a silent shva. Like oov-shochbecha in the shema, even though there's a meteg there. (Though sure, when many of us were in school, with our ashkefardi pronunciation, we learnt it as oo-vuh-shochbecha and didn't think twice about it!). In Sephardi tradition, maybe when there's a meteg with the vav shuruk prefix, they vocalize the shva.
Our instincts may say for HalVeeyim vs HaLuhVeeyim, that it's vocal but actually it's silent. It's following a patach. Sometimes its a patach with a meteg. According to Jacobson P267 there is a difference of opinion re whether it's vocal when there is a meteg. I think I learnt it as halveeyim, and I think that's what ashkenazi tradition is. Especially since vav shuruk with meteg in ashkenazi tradition is still silent shva.
P.268 Jacobson gives some interesting sourcse regarding the vav shuruk with meteg.
קֵ֔דְמָה Lev 1:16
shva (there), is preceded by a tsere, tsere is a long vowel. But there is primary stress on that vowel. So it doesn't match that rule(it doesn't match any rule , for it being vocal), so it's silent.
There is a point about nasog achor,that could shift stress, and if a verb or noun has a suffix then perhaps stress might shift or a vowel might shift.. and one should look at the word without the suffix and correcting also for any nasog achor. So, after reversing any effect of nasog achor or suffixes.. Then see if the long vowel preceding the shva, has no primary stress, if it does then it's shva na.
So for example, gemu-luh-chem(joel 4:4)..has a kubutz.. but without the suffix, it's gemul with a shuruk. Shuruk is a long vowel. gemulchem, is gemu-luh-chem, because one works out the shva as if it were a shuruk there.
And it's not a conjunctive shuruk.. a conjunctive vav shuruk i.e. a conjunctive vav followed by a Bet Mem or Peh, then that may actually be another shva rule.. (but either way there's an important note regarding it that i'll mention)
A Shuruk, that's not a conjunctive vav shuruk, is a long vowel.
We see that vav shuruk is in the middle of the word, so it's not a conjunctive vav.
We treat it like it's gmlchem With a vav shuruk, rather than a kubutz. So the spelling and vowel from as if it were without the suffix
so it says גְּמֻֽלְכֶ֖ם but re the shva it is read as גְּמֻֽןּלְכֶ֖ם a shuruk in the middle
of a word is a long vowel. it's there is a vocal shva on the lamed.
Sometimes a Meteg can indicate that a shva is vocal. The rule with the kamatz is clear(it makes the kamatz long).
But there are some complexities with the meteg, including some unknown complexities
From Jacobson, P289 it looks like, when a word begins with Vav and patach then yud with shva, there is or tends to be a meteg there, but regardless of that meteg, the shva is silent. I think i've seen this in Feldheim too, how they do things there. So Gen 34:20 VayDabru not VaYuhDabru. Gen 1:5 vay-hee not va-yuh-hee, Gen 5:6 vay-chee, not va-yuh-chee. (Many [casual] leiners i've heard do a vocal shva there but apparently really serious leiners would do a silent shva there matching what Jacobson wrote)
And this rule mentioned in Jacobson P289, and i've heard from a really serious grammarian too.
vav shuruk with meteg. The famous example is Oov-shochbecha in the Shema. Deut 6:7 (oov-shochbecha). There's a meteg there. Ashkenazim do a silent shva there. Maybe sephardim do a vocal one. The Sacks singers siddur mixes traditions up on that and has a vocal shva, there was quite a disagreement in connection with that, the singers sacks had oo-vuh-shochbecha. The Koren Sacks siddur, has Oov-shochbecha.
Another case and this one it's not clear from jacobson's book how one determines this. It says P289 regarding a meteg indicating a 'missing letter' (by which he means a word written in shorter form spelling than 'normal'). As in spelling can be ktav hasar(short) or ktav male(full). And he gives examples of a meteg with a chirik, where the chirik is not a chirik yud. And he's saying it should be treated like the vowel is long (chirik yud). And so any following shva is long. Similarly he gives an example of a meteg with a kubutz, and says that any following shva should be treated as long, as if it were preceded by a shuruk. Gen 48:20 Yuh-Shee-muh-cha (as opposed to Yuh-Shim-Cha), Deut 28:11 vehoh-tee-ruh-cha(as opposed to Ve-Hoh-Tir-Cha). Maybe the rule is the rule about removing the suffix though he doesn't mention that rule. P288-291 have 17 observations regarding "secondary stress" i.e. meteg.
A strange one is on the bottom of P288 Jacobson. Strange 'cos what Jacobson says there isn't reflected in Feldheim's tanach simanim Num 20:19 Bamsila or Ba-muh-sila Jacobson says is vocal. Feldheim tanach simanim has it silent.
And other than that, there are some unknown complexities with the meteg. Jacobson mentions different types of Meteg, including a normal one, and a "euphonic" one, and JAcobson says a shva following a euphonic meteg, is silent. Jacobson P293 quotes a grammarian Yedidyah Norzi who says that the rule for when a meteg is euphonic is not clear / we don't have a straight/valid rule to determine that, and if anybody were to pronounce a euphonic meteg as vocal then it'd some strange to the listener
Jacobson P.293 quotes Grammarian Yedidyah Norzi to say that they don't have a straight and valid rule to differentiate when a meteg is a "euphonic meteg". Whether he is indicating that this affects pronunciation or not, I don't know. And Jacobson suggests on P292 that a euphonic meteg is silent. And on P291 says that a euphonic meteg is a meteg in a seemingly inexplicable position.
Jacobson P289 mentions Gen 34:20 VayDabru (vav patach and meteg), but silent shva. And other examples. The vav patach prefix with meteg, followed by shva, then the shva seems to be silent. but apparently some disagree.
I guess that experienced baal koreh probably know the problematic or difficult ones from memory and have some decision based on a particular source. And they might also work the difficult ones out in their head, but given particular traditions.