I just looked up a 10' chunk of 3.5" standard (schedule 40) pipe online and it was almost $200. Sometimes it pays to cut down a longer piece if you can figure out how to move it (and you're willing to live with the pitting and rust).
I'm not sure I'd pick PVC over steel pipe. Sure, the surface is nice and smooth, but I wonder if it has the grab and torsional strength necessary to prevent twisting. PVC has all the right properties for pressure pipe, but the structural properties surely aren't the same as steel.
This pipe new, without welding is 9.00 or more a foot 9x9= eighty-one dollars; yes; materials a worth more new.;; and cost less old; and steel @ 3.0 id sched. 40 holds the big dish down, and they listed the types found all over the US from the 80's to the 90's...
There's technically no such beastie as 3" ID pipe. There's just 3" size pipe. 3" schedule 40 pipe is nominally 3.068" ID (although with modern tolerances it is probably bigger as the "thickness" of everything runs within a few thousandths of the absolute minimum).
Up to 12", pipe sizes are not based on actual dimensions but the one dimension that always stays the same is the OD so regardless of schedule, 3" pipe will always have a 3.5" OD. 3" XXH pipe has a nominal ID of 1.8" and an OD of 3.5".
The confusing part is that at >=14", pipe size is the outside diameter.