I'll speculate, and ask questions.
IS your TV a stereo audio output model? If so, your three wires are what you suspect: one left, and one right channel, and the third wire is the 'common' or return wire for the two channels.
I'm THINKING you mean that you want to listen to the headphones late at night, but want the TV's own speakers
to operate at all other times, and that what you want is to be able to listen to the TV normally, and then just flip
a switch and don the headphones at night?
If that's what you mean, it gets a little complicated, because when you insert the headphone plug into the TV headphone jack, the internal construction of the TV headphone jack is designed to disconnect the TV internal speakers, and route the signal to the headphones.
Is what I suspect you're trying to do, actually correct?
If so the problem for you will be getting around the physical construction of the TV headphone jack as it is presently wired.