Aluminium fellas.
3mm would probably be on its limit but it could possibly stretch to it.
Why don't you be proactive Giz and take a bit to try it out.
If it does it pay the man.
Rough rule of thumb is to add 4 swgs to the rated capacity in mild steel for ally and subtract 4 for capacity in stainless i.e. a guillotine (or shears, a folder, slip rolls etc) rated at 16swg mild steel will cope with 12swg (2.5mm) softish* ally and 20 swg stainless
The plate on the side probably says whether it's an 18swg or 16swg model, strength of the user doesn't factor into things with stomp guillotines because you've got nothing to pull or push against, especially if cutting up a full sheet that overhangs the treadles...
* Obviously something like 1050 or 3103-H14 is a chunk softer and easier to cut than some 6082-T6 etc
youd probably get away with it with a good jump, we had one at college the same when i served my time and it was amazing what it would go through with a few of the heftier appos bouncing on it...............