Gwaihir, Greatest of the Eagles

Legendary Creature — Bird Noble

Flying

Whenever Gwaihir attacks, target attacking creature gains flying until end of turn.

At the beginning of each end step, if you gained 3 or more life this turn, create a 3/3 white Bird creature token with flying and "Whenever this token attacks, target attacking creature gains flying until end of turn."

You create just one Bird token, no matter how much life you've gained past 3 life.
Gwaihir, Greatest of the Eagles's ability looks at how much life you've gained in the turn, even if it wasn't on the battlefield when you gained life. It doesn't care if you also lost life, even if you lost more life than you gained.
If you haven't gained 3 or more life by the time an end step begins, Gwaihir, Greatest of the Eagles's ability won't trigger at all.