We too, use our panel saw for most cuts. Just cut 11 4' × 8's down to 4' × 4's. Did 4 panels at a time. Had it done in about 10 minutes. If we're cutting an inch or 2, we still use the panel saw. On ocassion, we'll use our fletcher, but 6mm is hard to cut through and especially what you wanna cut. You're gonna be cutting against all the flutes. That's difficult with any hand tools and snapping. Don't you own a circular saw ?? If you'd set up the same scenario for using a knife, you could do the same thing with a saw...... and be done in 10 seconds, instead of scoring and scoring and scoring....... and beating it with whatever. With a shopvac, ya just clean up your mess in about 1/2 a minute. It will be accurate, straight and no worry of the knife going off track and in the end, just take a round file and run it one time down the cut and it deburrs it perfectly.
3 mm cuts absolutley fine on a saw, fletcher or even with a hand tool like a knife of some sort. Having the right tools, really helps....... or a friend with one.
Another thing is, maybe your supplier will cut your panels down for you, for a slight charge.