I've played 1,2 and I'm currently playing through 3. I've never finished Far Cry 2, because every time I've gotten anywhere close to finishing the game, my computer craps out for some reason (more than 1 computer and more than once per computer) so I believe there's some gypsy curses at work.
Currently I'm really loving how they did open world in Far Cry 3. In Far Cry 2, the major complaint of many people was that the enemy checkpoints always respawned, but in Far Cry 3 the enemy outposts have a sphere of influence where they spawn enemy patrols. If you take out the outposts, that area will start spawning your allies instead and you gain an extra home base. Gunplay is also more satisfying this time around, in Far Cry 2 the guns felt kinda powerless, but in Far Cry 3 I'm playing on the Hardest difficulty, and the foes seem to drop pretty fast. The game world feels more alive than in Far Cry 2, for instance there's the hostile wildlife that also attacks the enemies, there are sometimes allies and civilians driving or walking around the gameplay area. The random encounters are more varied than in 2, for example in Far Cry 2 if there was an enemy checkpoint somewhere, it would almost invariably be in the same place, whileas in Far Cry 3 the enemy checkpoints on the roads shift locations, there are enemies in the game world that occasionally have civilians as hostages and you're rewarded for rescuing them. The hunting and crafting system is pretty superficial, but it does help to give you some nice gear progression, and what I really like about the game is that you can't carry a whole lot of ammo with you per gun, so you're kind of encouraged to use different weapons. Taking out enemy outposts stealthily feels really rewarding and fun when you learn some of the game mechanics first.
I have something to complain about the game as well :
-The main character is a horrible cringe factory. This is pretty bad in a game where most characters are otherwise pretty solid, and generally the setting is fun to roleplay Jace&Deagle Nation in especially after you meet Ciadude420

but the main character is pretty cringey in a bad way and in contrast with the game mechanics.
-Getting the radio towers gives you free weapons, which kind of invalidates the whole money system.
-Quicktime events and gimmick sequences can be pretty bad.