well first question would be, is if your truck is a 4x4 or not?
If it is, I wouldn't recommend running a 3" spacer. It will ride terrible and it will be nearly impossible to align unless you get a axle pivot drop bracket and a drop pitman arm. My suggestion would be to get a 2" leveling coil instead of going with a poly spacer.
Now if youre a 2wd truck, I can't help you out because I don't really have any experience with 2 wheel drives