Strong, the Brutish Thespian

If a player has fewer cards remaining in their library than the number of rad counters they have when the triggered ability resolves, they’ll mill as many cards as they can.

Rad counters are a kind of counter that a player may have. They’re not associated with any specific permanents.

If lethal damage is dealt to Strong, its enrage ability triggers. Strong leaves the battlefield before that ability resolves, so you won’t put three +1/+1 counters on it. In this or any other case where Strong is no longer on the battlefield when its enrage ability resolves, you’ll still get three rad counters.