Combine multiple relationships into one subgrid

I've got several 1:N relationships between Account and Account called Preferred Vendor 1, Preferred Vendor 2 and Preferred Vendor 3.  I like this type of relationship as I can add in the information more easily than in an N:N relationship (where, for some reason, I can't get a + add new record on the subgrid?)

From a vendor perspective, however, it's a pain as I've got three relationships, so three different subgrids for Vendor 1, Vendor 2 and Vendor 3.

a) am I going about this the right way?

b) can I get the best of both worlds?

Thanks