After the three is to one split, for every one old share, there will be three new shares.
So number of new shares = 5000*3 = 15,000 shares
Since the number of shares increased three fold, the price per share will decrease by three fold.
So the price per share after split = 12/3 =$4
So, after the split, there will be 15,000 shares at $4 per share