Is it advisable to fit spacers on the rear wheels on a 3dr cosworth ive got 17"
on it just want the arches filled out a bit more

 

I've been running spacers on both my road and track wheels for 15 years worth no issues, just to get the offset right. Just don't go too big without using hubcentric ones.
Also, make sure you've got plenty of thread on the wheel studs. If not, they're easy to replace with longer ones