How do you program a Comcast 3-in-1 remote control to work a Symphonic DVD-VCR?

You may not be able to.

There are a few "hard-headed" manufacturers who make sets and equipment that have remotes with "oddball codes" and they cannot be replaced with the universal remotes. Some Symphonic units are like this. Comcast tech support may be able to help. Call them. They work for you. But brace yourself to accept that their unit - and none of the universal aftermarket remotes - will be able to be programmable to operate the equipment.

YES THERE IS A CODE FOR A SYMPHONIC COMBO TV TO PROGRAM A COMCAST REMOTE #20000 WORKS MINE JUST FINE.