Whats a great cheese blend for mac n cheese sauce?

two cheddars, gruyere, and pecorino

I use medium cheddar and sometimes add sharp cheddar. Add a few drips of hot sauce for added zing.
I enjoy broccoli as a side with a bit of lemon butter.

Agree with Britt McDaniel - go with your tastes.
For a wide range, look at this post (college student, extensive mac & cheese resume) and the virtues of each cheese she favors.
https://spoonuniversity.com/lifestyle/15-best-cheeses-for-mac-and-cheese

Use the two types of cheese you have already. If you like the cheese alone then I know you'll like the cheese together!