I live in an old 3 decker that was recently rehab'ed and turned into condo's.

There is a main stair way that runs up to serve the second and third floors. On each floor (main, 2nd, third) there is a light switch that when turned on, will turn on a hallway light on each floor.

We want to replace these switches with motion sensing switches - both for safety and convenience.

I'm no electrical engineer - can someone tell me if we need three way or four way switches?

Where you have 3 different switches controlling one load, they are called 4 way switches and have 4 contacts on them. I don't know if it will be easy to find 4 way motion control switches.

Actually this would be a 4-way system. However, the end switches would be 3-way switches with a 4 -way switch sandwiched between them. I wish I could help you but I have never seen anything but a single pole occupancy sensor switch. Another option would be to put another series of lights, possibly small unobtrusive ones, and use a mounted motion unit to operate them. One could add an override switch for them at whatever location they like.

Edit----

On further reading of your post... If the switch at each landing controls just that floors hallway fixture... then all you have to do is buy 3 switches. They do cost about 40 bucks eack but seem like they would do what you want. Leviton makes ones that should be available at any home improvement store.