Excel: Partial Matches in 2 Different Cells

Hello,
I've been mulling over this all afternoon and need some help. Here is what my sheet looks like:


K L
3 M1F RP/M1F
4 M1F Gray
5 CB RP
6 RP RP

What I need is the following- If 3 of the letters in K3 match L3, column M should say Match. If they do not, column M should say Mismatch.

Another layer to this formula should be that if column K and L should match and be either RP or M3L2, it should say Additional. So far, all I could come up with is
=MATCH(LEFT(K3,5)&L3,L:L,0)

Any help is GREATLY appreciated! Thanks!