Congrats on the new H90!

Input/Output 3 on the H90 will correspond to Insert 1. You can connect your guitar to H90 input 1, and connect outputs 1-2 for a stereo output.

It is similar to what is described here, just only using 1 mono insert: https://cdn.eventideaudio.com/manuals/h90/1.8.6/content/setup/two-mono-inserts.html

You can then use the routing page to determine where the insert will be, and save your Program with that insert routing. When that Program is loaded again, it will load with the correct insert routing.

Let me know if that clears things up.