the gigabyte board will do 3 with no risers. 1080ti 1080 ti 1070 mini
here are two builds on the biostar z170
The spacing on your Biostar Z170 boards is such you could probably get away with an Aorus as the slot 1 card (or that MSI "2.5" slot 1080ti since you seem to prefer MSI as a general rule).
cvsea - only way I'd consider adopting you is if you were a pretty gal. Sorry.
9-)