Probably reason why i'm currently unemployed, I didn't get it 555

Will give answer in a bit...Look at the four cards above. We know for a fact that each card has a letter on one side and a number on the other. Truthful Terry says: ‘Every card that has a D on one side has a 3 on the other.’ What is the fewest number of cards you need to turn over to find out whether Truthful Terry is actually telling the truth? And which ones?