I've noticed enemies buffing for extra attacks but I haven't found anything that lets me do that apart from off hand attack and lae'zel's extra attack buff.
Action surge will let you, as well as a haste potion or anything that gives you haste. Since drinking a potion is a bonus action you can drink the potion get the extra attack. Then next round you'll get 2 attack actions, use a bonus to pop your action surge as a bonus, and you'll get three. Careful though when you potion wears off you'll get lethargic.
