Murderous Redcap:

Question: Can you giant growth a redcap before its damage is dealt?

Answer: Yes, when a redcap comes into play it's triggered ability goes on the stack, so when you giant growth it, the giant growth will go on the stack on top of it's triggered ability, so the giant growth will give it +3/+3 before the triggered ability checks its power, thus dealing a total of 5 damage.