I'll make some assumptions here. First, I'll assume that you meant to write, "In act 3, what is Hamlet's reacton to what he has done?" That said, the other assumption is that you are talking about the biggest thing Hamlet does in the act, and that is that he finally kills someone. Hoping that who he stabs through the curtain is King Claudius, Hamlet finds that he has killed the eavesdropping Polonius instead. This is his reaction:
O me, what hast thou done?
Nay, I know not. Is it the King?
O, what a rash and bloody deed is this!
A bloody deed. Almost as bad, good mother,
As kill a king, and marry with his brother.
As kill a king?
Ay, lady, it was my word.
Thou wretched, rash, intruding fool, farewell!
I took thee for thy better. Take thy fortune.
Thou find'st to be too busy is some danger.
In short, he feels it's too bad that it wasn't the king dead there on the floor, and he is remorseless because he still has bigger fish to fry. He even blames Polonius for being the busy body that he was. No big deal. Then Hamlet wastes not time in ripping into his mother and tells her of how bad she has been to have married her first husband's murderer.
The act ends with Hamlet's acknowledgment that his killing of Polonius will set a number of things into motion, and that's just fine with him:
There's letters seal'd; and my two schoolfellows,
Whom I will trust as I will adders fang'd,
They bear the mandate; they must sweep my way
And marshal me to knavery. Let it work;
For 'tis the sport to have the enginer
Hoist with his own petar; and ' shall go hard
But I will delve one yard below their mines
And blow them at the moon. O, 'tis most sweet
When in one line two crafts directly meet.
This man shall set me packing:
I'll lug the guts into the neighbour room.
Mother, good night. Indeed, this counsellor
Is now most still, most secret, and most grave,
Who was in life a foolish prating knave.
Come, sir, to draw toward an end with you.
Good night, mother.
All in all, Hamlet seems to have been invigorated by what he has done and is rather pleased with himself and with what might come next.