2000 Chevy Truck crate engine with low oil pressure

I'm sorry to hear that, you have some very serious problems, until I get some more information, I know just about as much about it as you, I will make you a list of some of the questions I need to know.
1. Did you have any head work done? and if so what?
2. was the engine rebuilt at a shop? or do you do it? or one of your friends?
3. Did you use the same block? and if so how many miles did the engine have on it? and what did you do to the block? did you use new pistons?
4. did you put a new cam and lifters in?
5. what kind of oil are you using?
those are just a few questions I need to know and anything else you can tell me.
Now to answer your question about the cam "I was told the installer was suppose to break in the camshaft" that is untrue, the cam shaft breaks in on first start up, after about 15 minutes of running, and the assembler should have used a special grease, to lubricate the cam for Break-in. now to answer your oil pressure problem, the vehicle only holds 5(five qt.) the pan holds 4 (four) and the oil filter holds 1 (one) so if you had to add 3 (three) that means you only had 1 (one) in the pan, I'm sorry, that's not good enough, what you have probably done is damage the bearings, not necessarily spun one, if you spun a bearing you would have no oil pressure, but if you've damaged them, you have cut the life of that engine, by over 1/2 (half), what I suggest you do, it is drop the oil back out of it, and the filter, and installs straight 50W weight, if your oil pressure increases, you have damaged the bearing.