I did 3U maths at school and I went alright at First year maths. I seem to think that it is similar in some areas to 4U maths. Well, as much as I could pick up from people who did 4U, anyway.
First year generally has a lot of subjects that are done just to give you a background in the area (Physics, Chem, Manufacturing, Design, Computing, Materials). I really see the other subjects I did (Eng Mechanics, Mechanics of Solids, and maths) as the core subjects that will be built on in second year.
Basically if you're not afraid of some hard work you'll be fine. One thing I've learnt about Uni, even moreso in Engineering, is that you're marks are pretty much proportionate with the effort you put in.
And yes, I've just completed first year Mech Eng.