First, thanks in advance for your time.

I have two 3-way switches in my home at opposite ends of the house that control two different flood lights outside on the same circuit. I bought and hooked up a motion sensor to one of the floods thinking that when it is triggered it will turn both sets of floods on and off. Not the case. Only the one connected to the motion sensor comes on and turns off. How can I get both sets of floods to turn on with on motion sensor?

Thanks again.

I'm not an expert in electrical, but I know this is where it gets tricky:
The motion sensor you've installed is, most likely at the light that works the way you want it to. You need to have a different sensor "hard-wired" in to both lamp's circuits to have them both work from a one-sensor switch.
You should contact a local licensed electrician, or two, and ask for references. Take your time.
You may get some insight from a LOWES/HOMEDEPOT type store. Good luck!

If your 3-way switches work two different set of floodlights you need to get a motion sensor that can run parralled. One of the flood light boxes has two set of black, white and ground wires and the other should just have one set. With the switch off, disconnect the two black wires and separate them making sure they are not in contact with ground. Turn the switch on and determine which is the "live" or "line" wire. Once you have determined that, turn the switch back off and wire the black from the motion sensor to the "live" wire and wire the flood light at that location and the other black wire (load to the other fixture) to the red wire.