It'll be much cheaper for you to replace one of the R15's with a dtivo, they're still around.

You're looking at a RF universal remotes like the mx-350 or higher-end remote with IR addressable feature. You setup 3 RF only devices on the remote, assign each a different IR output on the separately sold RF basestation, the mrf-250, tape up the IR repeater cables to the sat receivers. So only the IR commands from the mrf250 each the receivers.