You can travel from Terminal C to Terminal B1/B3 on the shuttle bus, which you can access near gate C71, but you cannot go back in the opposite direction by airside bus. The bus drops you off at the bottom of a stairwell and when you go into Terminal B, you arrive through an unmarked door that says emergency exit only. No one is allowed to go down the stairs and the door will either be locked or guarded by TSA at all times. This is probably due to the US policy of no sterile transit for int'l flights, and thus, they don't see a need for people to be able to go from the sterile area of Terminal B to the domestic gates entirely airside, since arriving pax have to clear customs and come out of customs landside. NOTE: Terminals B1, B2, and B3 are NOT connected airside, so if you need to go to Terminal B2, you will have to exit and go back through security.