I have a motion sensor activated floodlight outside my back door. It is also attached to a switch inside the house.
For reasons I won’t go into I don’t want the motion sensor on this light anymore. I just want to be able to turn it off/on with the switch.
I took the motion sensor off the light and cut the 3 wires going to it (red/white/black).
Which of these wires do I attach to eachother to “close the loop” per se?
You can’t tell without seeing what attached to what. A logical guess could be made, but I wouldn’t do that here.
It sounds like you never took the light off, because the sensor normally is connected by electrical nuts to the light. Remove the light and look in the electrical box.
My guess would be red to black, but be prepared for and Earth-shattering Kaboom if I’m wrong.
Best bet is to remove the entire fixture and look inside.
Many (most?) of these sensors work so that you do not need to do anything at all to have what you want. They usually work something like this:
Switch off - light off (obviously
Switch on - light on automatic detect
Turn switch on for an instant then off for an instant then on again: light ON regardless of detector.
Where’s the Kaboom? There was supposed to be an earth-shattering Kaboom! 
This (red-to-black) is almost certainly correct. It’s very rare for the red wire to not be the “switched” hot. I’ve played around with quite a few of these and would feel very confident wiring it that way. If it were me though, I would reconnect them because what sailor says is correct as well. I haven’t seen one yet that will not work as he describes. They’re probably out there but I haven’t encountered one yet.