You can do a credit authorization in advance for their rooms, and only authorize room plus tax. They'll have to put their own card down for incidentals.

You will get $ credit for three rooms, and nights credit for one room, assuming you are at a full service Marriott/Renaissance/etc. Some of the cheaper properties will give you nights for all three rooms, but this is an error and not something you can count on or request.

There is one more option for getting points on all of the rooms if there are others attending the event who would be willing to book into your block. Call sales and create a room block (minimum 10 rooms) and you will get $ points (but only 3 per $) on all rooms, plus 10 nights credit for hosting a "rewarding event." A happy benefit of this is that your guests can get their own MR points for their rooms too, and you can get MR points on three of the rooms you pay for in addition to your hosting points.