[cencio]

I don't remind.
After this bid: 2nt- 3c- 3D. 3d is always transfer or it is an hand with diamond

[Stephen Tu]

It's up to partnership agreement. It often doesn't get specifically discussed; fortunately the auction is rare. I'd default to the agreement over 1nt-(2c)-2d, which is usually transfer in my area of the world.

Over non-club bids, bids are usually *not* transfers, just natural and forcing at the 3 level (non-forcing at 2 level).

[TylerE]

And of course it depends on what 3♣ means! Even if you normally do play transfers if 3♣ is both majors...