The relation between 2SAT and 3SAT

Question

Show that proving 2SAT is not NP-Complete would prove that 3SAT is not in P.

Or eqivalently -

Show that proving 3SAT is in P would prove that 2SAT is NP-Complete.

I can see there is an easy reduction from 2SAT to 3SAT and it seems like it is the key here, but I can't quite prove the statement above. Can anyone help?