3C is Gladiator, requiring opener to rebid 3D, and responder can pass or bid a weak 3H or 3S; however responder may have a poor 6 card minor to K or Q or so, and can indicate this by continuing with 3NT or 4D. Opener can then choose to play in NT, or in responder's suit.
3D is a Stayman-like relay for stronger hands; opener bids his lowest 4-card major, or 3NT with none. After the initial response to 3D, responder can make a further relay of 4C; in response opener can:
3H and 3S are forcing and unlimited;
4C and 4D are Texas transfers to Hearts and Spades, respectively;
4NT is Blackwood