I just purchased a corn plant 2 days ago. It was completely green. Now 3 of leaves are yellow and one of them ius partially brown. I have not watered it yet. Any ideas what the problem is?

Check the soil for dryness or moisture. Also when you purchase corn plants like this one from a Home Depot or the like they are not always A grade. You also might have the plant is low light. Try putting it in a higher light area and check on the moisture of the soil. The one you have doesn't take much water, but water well then let it be. A corn plant has very fine roots and you can easly drowned them.