i was looking to put a lift on my disco to. i herd all u have to do for a 3 inch lift is buy the shocks and springs. is that correct?

 

and how big of tire can u fit under a 3 inch

 

285/75/16 for sure, any bigger will cost you dearly in the form of gears, axles, lockers, wheel spacers and a fair amount of additional trimming.
Why do you want to go that large, what sort of wheeling are you doing and are you ready to loose at least 3 MPG?

 

you will also have to put extended brake lines and watch reroute your ABS sensor lines.