Whats the source for taking 3 separate sips before gulping a drink down?
The Shulchan Aruch (OC 170:8) writes that the proper way to drink a regular-sized drink is not to down it in one gulp (too glutinous) nor to sip it bit by bit (too pretentious), but rather to drink it in a couple of gulps (juuust right). The Aruch HaShulchan makes it clear that the general idea here is just to drink like a civilized person, however that may be for the drink that you're drinking (for example, hard liquor may be acceptably sipped).
In order for a bracha acharona to be made on a drink, at least a revi'is must be drunk within a normal time span. This is fine regularly, but when it comes to hot drinks which are difficult to gulp, if one wants to make a bracha acharona, he would have to try and drink a revi'is quickly. How quickly? The normal amount of time it takes to drink a revi'is (כדי שתיית רביעית) (Shulchan Aruch OC 612:10). There are various opinions with regard to how long this is and in how many sips one may accomplish it. According to the popular halacha book V'zos HaBracha (p. 227) one who drinks a revi'is in three consecutive sips is obligated in a bracha acharona.
Now, "taking 3 separate sips before gulping a drink down" would seem to have no benefit according to the above. If one will anyway proceed to gulp the drink down, there is no need for the sipping to obligate one's self in bracha acharona; in fact it may violate the Shulchan Aruch's view of proper drinking etiquette if it is a drink that should be gulped normally rather than sipped pretentiously. If one takes three sips of a hot drink (or other commonly-sipped drink) and then proceeds to sip at his leisure, then that has the benefit of obligating a bracha acharona, so long as he is able to imbibe a full revi'is (~3.8 fl. oz. according to the Chafetz Chaim) in the process.