All HUD stats are calculated based on opportunities. In PT4 you can put your mouse over the stat, and then it will tell you the sample size, and generally this is the first number, you should even look at, because it will tell you, how reliable that information is. If a certain number is 50%, but its one out of two, that does not really tell you a whole lot.
For someone to 3-bet, there must have been a 2-bet (also called open raise) in front of him, so only those hands are counted, where this was the case. If noone entered the pot other than the blinds, or if people limped in, you can only 2-bet. If someone already 3-bet, you can only 4-bet, so again its not counted.
So yes a 10% 3-bet does mean 10% of hands. However it does not have to be the top 10%, since many players will use a polarized strategy, where they 3-bet for value, but they also 3-bet some hands, that are actually not strong enough to call. Also most players will have different 3-bet ranges based on their own position, the position of the opener and other information like the HUD-data of the opener.
I dont really feel, I can explain completely, how aggression factor and aggression percentage is calculated, and it might even vary a bit from program to program. But basically these stats tell something about postflop tendencies. High numbers mean, someone is constantly betting or raising and rarely calling, low numbers mean the opposite.
Its not quite as simple as that, but its difficult to maintain very high aggression numbers, if you are never
bluffing, so everything else being equal high numbers should entice you to do more
bluff catching, and low numbers should entice you to do more hero folding.
Take the classic drooler fish, who always limp and call, and whose main strategy is to try to make a hand and then get paid. Such a player will have low aggression numbers, and if he called you preflop, called you on the flop, called you on the turn and now suddenly raise you on the river, do you really think, he is doing that as a bluff?