Behind my bathtub, the pipe that goes from the shower to the faucet has a leak at the elbow. It is a copper pipe. We tried using flux and safe flo silver lead free solder with a propane torch. The joint is leaking at the solder point and there are three pin holes in the elbow, but there is still some water in the pipes. The solder is not melting to adhere to the joint. How do we fix?

If there are pin hole in the elbow than you have to take that elbow out and solder in a new one.

You cannot solder with water inside pipe. Close water to the house, open hot and cold faucet in fixtures below this one, let water drain - and only then proceed with soldering. Clean both ends of pipe and inside of new elbow with steel brush, apply flux on ends of pipe, slip pipe into the elbow, apply heat from bottom up. Once pipe is hot, apply solder.

Milo is correct you can't sweat any fitting with the water inside the pipe, follow his instructions, I am adding the link below for the Sharkbite fittings I would suggest and if Milo agrees would replace the existing elbow with the Sharkbite elbow.

Sharkbite Fittings | Instant Push Fittings for CPVC, Copper and PEX Tubing SharkBitePlumbing.com

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there are three pin holes in the elbow,

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Follow the expert advice already given to replace the elbo.
The pinhole leaks bother me. They might be a indication that your system has electrolysis that's eating up tour pipes. Check a copper pipe back to earth ground with a AC meter. There should be no reading on the lowest setting. This should be done ASAP. Good luck, Tom