In triple DES you take the block and then:
Send it through DES once with a DES key k1
Send the result of that through DES a second time with a DES key k2 (for this second time we generally run DES backwards; in decrypt rather than encrypt mode, however, that's not important for this question)
Send the result of that through DES a third time with DES key k3.
The result of that is the result of the 3DES block operation.