You may heard, that Jax is the most over powered champion in LoL, thats right. You have with Jax a stun attack, you can dodge and jump to your enemy. In a 3v3 you don't have a chance against him you're enemy knows how to use his attacks.

By the way he has the best skin ;)

As you probably know your passive brings you 3 HP for every physical attack damage and 2 HP for every abillity power. So you don't need any tank items.
NINJA TABI 25 armor, 12 dodge These boots are very important for your Counter Strike
HEXTECH GUNBLADE +40 Attack Damage +70 Ability Power +15% Life Steal UNIQUE Passive: +20% Spell Vamp UNIQUE Active: Deals 300 magic damage and slows the target champion's Movement Speed by 50% for 3 seconds (60 second cooldown) This items bring you some ability power and some attack damge (very important)
GUINSOOS RAGEBLADE +35 Attack Damage +45 Ability Power UNIQUE Passive: On basic attack or ability use, increases your Attack Speed by 4% and Ability Power by 6 for 5 seconds (effect stacks up to 8 times). Again it brings you a little bit of both
NASHORS TOOTH +50% Attack Speed +55 Ability Power +10 Mana Regen per 5 seconds UNIQUE Passive: +25% Cooldown Reduction It brings you some attack speed , again ability power and some very nice 25% cooldown reduction
INFINITY EDGE +80 Attack Damage +25% Critical Strike Chance UNIQUE Passive: Your critical strikes now deal 250% damage instead of 200%. This item is for your attack damage
ATMAS IMPALER +45 Armor +18% Critical Strike Chance UNIQUE Passive: Gain Attack Damage equal to 2% of your maximum Health. Item brings you atack damage from your HP you gained with the attack damage and ability power

At the beginning you need you're Leap Strike for fast minion killing and doining some damage to your opponent. BUT NEVER TRY, UNDER LEVEL 6 TO KILL YOU'RE ENEMY ALONE!!!
And thats the way you should skill you ability's:

MASTERY TREES
1/1
Summoner's Wrath
3/3

4/4

1/1

4/4

1/1

1/1

3/3

4/4

3/3

4/4

1/1