I have 2 bedrooms that I am trying to install new light switches in. They both had dimmer switches in, now I just want regular flip switches.

The problem is that Room A seems to want to be a 3-way switch with room B. There are 3 black wires in the electrical box. Right now, with the dimmer switch hooked up, 2 blacks are attached together to a single black on the switch, and the last black is hooked up to the other single black on the dimmer.

Room B only has 2 black wires, so it's a single pole switch, correct?

I want room A to have no control over room B. How do I do that?
Can I put 2 of the 3 wires into a single pole switch and just cap the 3rd wire?

To turn on lights in room B does that single pole switch always do it or does the 3 way switch need to be in a certain position?

Do the switches from both rooms control the same light(s).
If they control separate lights, and room A is a 3 way. Then:

At the location you want to eleminate, connect the common with either traveller, and cap the 3rd wire.
Then go to the switch location and connect your single pole switch to the wire that is on the common, and test to see which of the travelleres you need to use. Cap the extra wire.

To answer your question, using 3 way as a single pole. Use the 2 screws on the same side of the 3 way switch. And install switch, so when installed, the 2 screws with wires are on the right side, which is the same as a single pole switch.

If I understand correctly you hqve a dimmer switch with two black wires in Room A. In that switch box you have 3 black wires.

Remove the dimmer switch. Mark the black house wires A, B and C.
Connect wires A and B with a wire nut.

If the light in room A is OFF and the light in room B works properly, reconnect the dimmer switch.
If the light in room A is ON, disconnect wires A and B, then connect wires A and C. If the light in room A is ON, disconnect and connect wires B and C.

In other words find a combination of two wires that make the light in room A OFF and the light in room B work properly.

Then reconnect the dimmer switch, connecting one black wire of the dimmer with the single black house wire and one black wire of the dimmer with the two combined black house wires.