Will an XX mech work with an old 3 speed XO front shifter, does anyone know?

Maybe, are you running it on a double?

Yep. Triple mech tends to sit a bit high and looks messy.

As long as you run the rings in the inner/middle position, or have a specific 2×9 chainset then yes it should work. Expect a shorter throw though, it may well rub in some combinations.

Or use a road mech.

Edit: or move your standard mech down…