So I got DS3 on the PC but played ME3 on xbox because that's where my saves were. I was wondering if there is any way to "trick" the game into thinking I have a ME3 save on PC without actually having ME3.
Sounds like the easiest legal way to do this would be contacting a community manager at Visceral and ask for a code. You might get a dumb, corporate answer like "Sorry, the only way is to buy and play the wonderful entertainment product that is Mass Effect 3 on Origin!" but it's worth a shot.
Do the different suits ACTUALLY do anything? I didn't seem to think they did since your upgrades this time are just generic upgrades. I think Brad even said they don't really affect anything, but each suit has a description about what it's best suited for, is that all just flair text to make it seem like you should changes suits? I rocked the N7 armor for a while but then switched it up for the sake of it. After having N7 armor in DA:O and Kingdom's of Amalur, I was kinda over it.
Do the different suits ACTUALLY do anything?
Nope, the only difference between each suit is purely a cosmetic one. Even during that one time when the game heavily suggests you put on the Arctic suit, so long as you simply change your suit, regardless of what suit you change into, then you can still continue onward.