Can I connect a 3 Pole on/off switch to 1phase 120v?
The hot (line) from your 120Volt supply will connect to any one of the three poles in your disconnect switch (unless it has guidance for a particular pole for single-pole use in its labeling)

The neutral and ground connections pass through, with the ground connection  bonded to the case, assuming it's a metal-cased switch.

As commented, you may have some issues supplying more than 20 amps of 120V without some custom wiring, and it's often easier (for the many VFDs that will happily take 240VAC input - or swap yours if it's 120VAC-only input) to switch to 240VAC if you are facing custom wiring anyway.

In a standard line cord, neutral is White (or gray) (USA Standard) or Blue (IEC standard, not uncommon on imported equipment even if intended for the USA market.)