Okay, I've read several Q's and A's here. I guess my situation seems unique, so I'll ask.

I have hot water everywhere else in house.
I have decent cold water pressure in shower
When I turn knob to 180 degrees shower goes from good cold pressure to trickle cold w/ no sign of hot.
When I pull off plate, etc.. I see 3 holes, the top 2 have a little rubber fitting w/ a spring set in... the bottom does not have the rubber and spring.
There IS hot water coming out of one of these holes!

So, I know there is hot water getting that far...

Notable mention: I just got the place out of foreclosure and have the feeling that the previous owner might have screwed w/ whatever he could in vengeance. So it might be tricky...
One thought is that the 3rd hole needs the rubber and spring.. but I'm not sure.

Thanks for any suggestions

I think you have pressure balanced faucet. Chances are that the balancing piston inside the cartridge is stuck in one position - pushed all the way to the hot water side. It usually happens when faucet is not in use for extended period. Solution: either buy new cartridge or see if you can dislodge that pressure balancing piston loose.

Also, you could have dirt in the body of the faucet...

OK, thanks.
I guess I'd like to try to dislodge before replacing.
How do I do that? Is this the pull everything off and turn the main water back on for a good jolt of pressure trick?

Shut off the main, remove te clip on the top of the valve body by prying straight up tehn extract the cartridge. Inspect the cartridge and clean. Hold a towel over the valve body opening and have someone else turn the main ON and Off quickly to flush the lines. Grease with plumbers gease or silicone jel and reinstall, better now?