It seems like Moshe warned Pharoah of every plague except the 3rd (lice), 6th (boils) and 9th (darkness). Why is that? Is there something special about these plagues?
The Maharal (Chapter 56 of Gevuros Hashem) points out that the plagues follow a pattern, split into units of 3 - the first of each group (plagues 1, 4, and 7) are preceded by a warning to Pharaoh issued by the Nile. The second of each group (2, 5, and 8) are preceded by a warning issued to Pharaoh while sitting on his throne. The third of each group has no warning.
The Maharal explains that the plagues are in 3 groups, working their way up through creation, and each group within itself increases in intensity. The first group is in the earth itself - water to blood, frogs from the water, and the dust being turned into lice.
The second group is on the land - plagues against man and animals, again with the same progression of intensity.
The third group is the sky - hail which came from the sky, locusts which blocked out the heavenly bodies, and darkness which completely blocked out the light of the sun, with the same progression.
The plagues were split into these groups of three to split them into their divisions of which part of existence they targeted.
Within each group, the first had its warning by the Nile, outside of Pharaoh's seat of power. The second was closer to home, with the warning at the throne itself. The third did not even get a warning. This, explains the Maharal, showed the highest degree of control, that Hashem didn't even need to warn him before He struck him.
The Maharal offers a second approach, in which he explains that in each group of three plagues, the third was a synthesis of the first two. Therefore, the third did not need a warning, because there was nothing "new" about it - it was a composite of the previous two.
The final plague was a "grand finale" which did not get an immediately preceding warning, but was the culmination of the entire display.
Rabbi Hirsch points out several patterns, but recall that God had told Abraham:
"Your offspring will be a foreigner in a land not theirs, they will be enslaved, and tormented. The Jews experienced three types of Egyptian behavior: "you don't belong here", "you are less than us", and inflicting pain.
Each trio of plagues therefore followed the same pattern as punishment:
Disorientation (really weird sci-fi stuff -- the Nile turning to blood, jungle animals running amok, hail with either lightning or fire, depending your translation), which began by rudely "bumping" into Pharaoh while he was doing his business early in the morning at the Nile;
Inferiority (humans losing out to frogs, germs, and locusts), which began with Moses walking right through Pharaoh's palace security.
And physical pain? (Lice, boils, immobility due to darkness.) The goal was to inflict pain, therefore they were unannounced. As things hurt the most when you don't see them coming.
Rav Hirsch in chapter 7 at the end of vs 15 writes 'As we observed above, the first two plagues of each group were rather more to teach a lesson, as only the third was really a punishment, above that was why the two first ones only came after having been announced beforehand.'