Rulings

You choose up to three total targets from among other creatures on the battlefield and creature cards in graveyards. You may choose zero targets.
You choose the targets as part of putting the "enters" trigger on the stack. Because that ability includes "you may," you choose whether to exile the targets when the ability resolves.
If a creature targeted by the "enters" ability dies before that ability resolves, it will become an illegal target even though it may be a creature card in a graveyard when the ability resolves. It won't be exiled.
If Angel of Serenity leaves the battlefield before its "enters" ability has resolved, its leaves-the-battlefield ability will trigger and do nothing. Then the "enters" ability will resolve and exile the targeted creatures and/or creature cards indefinitely.