Can’t speak specifically for the wiring of the Strymon 3 button, but the h90 should work fine with a normally open switches connecting:

tip to sleeve

ring to sleeve

tip and ring to sleeve

assuming the Strymon does this, I would confirm you’ve correctly configured the exp port as a 3 button switch and not an expression pedal per:

https://cdn.eventideaudio.com/manuals/h90/1.12.5/content/system-menu/io.html