Pls say your pros and cons to these champs

They are both very solid picks for ranked. Both have a high skill cap.
What i can say to cassiopeia:
+ you never get bored of playing her
+ very stable damage with Q + E combo.
+ Aoe stun can be cast out of nowhere with help of flash.
+ You can scout bushes with noxious blast and miasma. This can help to avoid ganks
- Champions like Xin Zhao, Pantheon, trynda and TALON are really annoying when they get in melee range.
- If you do well, people start to babysit mid and have you first kill on sight.

and who do you think is better for 3v3???

Major con of Ahri are skillshots - missing them will render you worthless. Also if you can't score a kill with single burst you will be at disadvantage to those champions with sustained damage (Cassie, Swain, Malza etc.)
Major con of Cassie is if she will miss E (in meaning that it won't hit poisoned target) her DPS is drastically lowered, but it is compensated by the fact that Twin Fang works with any poison in game (from Singed, or Teemo for example).

They are both good. But if you like nice dmg, choose Cassi. She has nice Ulimate and E <33. If you like more mobility and CC choose Ahri. Her E is pretty nice, if you want get him under the turret (Free Kill) =PP.

I'd prefer ahri. cassie has a bigger nuking potential but you would better towerdive or leave with ahri's ulti than with cassie and i prefer survivability potential more than damage output cos if u run for a kill but something goes wrong, you cannot leave it but you will die

I agree that cass lacks mobility, and that makes her a very unfun character for me. I love being able to move around like a boss, or being able to pull of silly escapes (538 movespeed blitz coming through! and I could still tune it higher if i wanted! 3 sec riftwalk kassadin, try to catch me!)

For 3v3 I would rather pick Ahri, Cassiopeia is incredible on TT if fed, she can easily get triple kill wandering alone around the map, her ultimate can win games, she clears minion waves like devil, and it's impossible to run away from poison.
But.
On a solo lane half decent team will gank hell out of her, she has no real escape mechanics (flash over the wall is not very spammable) and as was said - she really lacks mobility, and before lvl6 she can be ganked successful easily. After lvl6 also.
Babysitting on duo lane with Cassio means almost no farm for lane partner, she eats everything, so there is no option like that available. -.-
Talking abour raw power Cassio leaves Ahri far behind, but in survivability... That is one thing, another is her cooldown. Her only CC - 2 second long stun is available every 2 minutes, even with cd reduction it's tremendous amount of time on TT. CC Ahri (charm) is up every ten (or so) seconds. You'll never notice that you've missed.

Going back to the question - who is better for 3v3? Absolutely Cassio, but Ahri is more reliable pick, maybe she can't melt faces like Cassio, but she is harder to counter (true damage ahead) and kill. And is one hell of annoying champ.

Choose Ahri. With Cassiopea you defeat me

I really enjoy playing with ahri. High mobility, great 1v1 nukes, great champion. Not so easy to play, though cuz of her two main skill shots, but i love her cuz of that :P :P