I have two switches with the following cables in each box:

Switch A: Red/Black/White, Red/Black/White
Switch B: Red/Black/White, Black/White

I got cute and replaced the original [A] single switch with a double switch, without writing down the original wiring. Now I realize that I wanted to keep the original single switch.

How do I properly wire A? Which wires do I pair up with a wire nut and which go to the switch? Thanks!

Not enough information here. Sounds like you will have to call a professional. It is usually a fairly straightforward job.

EPM

You titled your post 3-way switch. You then refer a single and a double switch. What do you mean by single and double switch.
How many terminals (screw) does the original switch have. What does the switch(s) do? Switch A is in one box and switch B is in another, correct?

Switch A had 6 wires?

Na, switch A is a 4-way, 4 terminals with a neutral or common return leg running through box. Switch B is a 3-way. Bet you.

He lists 6 wires going to switch A. We need some clarity!