I find totally useless using these kind of assassins 
when enemy adc builds 
.
and midder builds 
What should I focus?
I should wait that the enemy used these item leaving my mates 4v5 for a great part of the fight?
P.S Kha has some chance to kite, but the other 2.....
I should wait that the enemy used these item leaving my mates 4v5 for a great part of the fight?
Precisely. Assassins whole job is to flank and wait for opportunities to burst carries. If items' triggered abilities are something in the way then you've got to wait for them too. Or bait them out in the case of active abilities like Zhonya's.
Build Hydra and splitpush.
rengar isn't even an ad assassin anymore, he build mostly brusier.
If they counter build you like that there's not a lot you can do if those items passive are available. You need your team to do something about it before hand or pick a new target.
You try to fight the cancer riot has created then soon realize ad assassins are just worse than mages right now and swap to being a mage main. Same burst, less risk, and items don't dumpster you for free.
Talon has the best kiting in his W.
