patricej

i have used the Tri-Rec set many times.
they are very useful if you need those shapes in blocks.
if you follow the instructions you will get great results.

i had never seen the Tri-Mate before you asked this question.
i don't know how often you'd need that shape but i'm sure it would be useful if you did.

i am guessing that if you sewed two Rec pieces together across the bottoms you could get the same shape the Mate makes.