You would just need a singlemode device to drive out of Bldg A (offsite). Then a singlemode and a multimode port at Building B, one to drive the inbound SM from Bldg A, and one to drive the MM outbound to Bldg C. At Bldg C, you'll just need a multimode port.
I stay away from media converters, or anything else that's not SNMP manageable, wherever possible. First, it's a point of failure you can't see easily, next it's just sloppy networking, unless you are backed into a corner. You should have no trouble finding a 100BaseFX or Gigabit Ethernet piece of equipment that can be outfitted with the necessary ports. Max number of devices: 3, one for each building.
