installing shut-off valves
First we need to be clear on what you have there now. You said it's 3/8" threaded pipe, did you really mean it's 3/8 tubing? Or it it really pipe. 3/8 threaded pipe measures about 1/2 OD.
First you have to figure out how to shut the water off at the source, open the faucet to relieve pressure, then remove the supply line, remove the fitting that's on there now, clean off the pipe threads, apply teflon pipe dope, then Install a 3/8 FPT (female pipe thread) to 3/8 compression right angle 1/4 turn shut off valve, no extra adapers needed.