Depends on how you want the switching to work, if you want it to work the same as the current set up, except with a motion 3 way switch, just replace the current 3 way with a motion 3 way. If you want to have the other switch as a "master" (where the lights will only turn on if the master switch is on) you can use a regular motion sensor and then a regular switch. Here's a couple diagrams to understand 3 way switching and "master" switching:
3 way
"Master" switch aka 2 switches in series
Don't worry about the power source in the 2nd photo being a battery. Same principle applies.