Inexpensive Tri-Wire Cables

Try DH Labs Silversonic. Its relatively inexpensive and can be done it three seperate runs, terminated together at the amp end. Would be a little bulky.

I bought some custom DH Lab tri-wire cables from Jeff Delman of Value Audio. He advertises on Audiogon. He's very helpful and has a wide selection of terminators. Search on DH Lab and look in the classified area.

Analysis plus has cables in different guages that work great together. If you contact them through their web page analysis-plus.com they will custom factory-terminate whatever you need.

You could even schuffle with different single-wire factory terminated cables or mixing a single-wire with bi-wire for a "larger room of experiments" but it's truly pain to connect two or three spades onto one binding post(bananas cannot be connected a few onto one binding post).
I think it makes a sence to have a different gauge and grade for upper binding post.

Alon makes a tri wire arrangement for their speakers which is not very expensive.