Im no professional but i would probably set it at 85-95 amps with the 2.5 rods?. The 3.2's would be a bit of an over kill on 3mm stuff i would of thought.
If you've got vertical down rods (quick freezing flux, Oerlikon Supercord or similar) about 120-130 amps and move fast for a nice concave fillet. Just don't let the flux get ahead of the pool. Not a suitable tecnique for thick plate or multi-pass welds, but fast and strong enough for 3mm. W/A