You choose two targets for Evolutionary Escalation’s ability as it’s put onto the stack. No player may cast spells or activate abilities during your turn before this happens.

If one target becomes illegal before Evolutionary Escalation’s ability resolves, you’ll put three +1/+1 counters on the target that’s still legal.

If there aren’t two legal targets for Evolutionary Escalation’s ability as it’s put onto the stack, it’s taken off the stack and no creature gets +1/+1 counters.