My opponent runs out of cards during pegging, I have 3 left, can I get a go and then last card?
The American Cribbage Congress - Rule 7. The Play clearly states.
Rule 7.2 Scoring
When a player's opponent has no more cards or calls "go," the other player may play all playable cards in succession and announce the points scored before pegging the total at one time.
So yes, after your opponent runs out of cards you shall play our all your remaining cards, in any order you desire, scoring all points thereby accrued.