if i kill viktor leader and 2 other units. does hes passive trigger or all die together?

No — Viktor's ability does not trigger.

Viktor, Leader dies in the same game action as the other two units, so he's no longer on the board when the trigger condition is evaluated — a permanent's triggered ability can only fire if its condition is met while it's still in play (Rule 383.2.c.2). Units killed simultaneously don't "see" each other die.

This is true whether they're killed by a single "Kill 2 units" spell or by lethal damage in the same Cleanup. The only way Viktor's ability triggers is if the other units die before him in a separate game action while he's still alive.