A B737-300 Captain reported conflict due to attempts to board passengers with less than the required number of flight attendants on board.

Narrative

Upon arrival; only one Flight Attendant met the aircraft. We learned from this Flight Attendant that the other two Flight Attendants had received a short callout and would be at the gate five minutes prior to our push time. The first Flight Attendant specifically told me that she did tell the Agent that we were holding for the two Flight Attendants. Less than ten minutes after my conversation with the first Flight Attendant; another scheduled Flight Attendant showed up directly followed by a fourth jumpseater.As I began to brief the two scheduled Flight Attendants in the forward galley; the Agent brought two extremely obese passengers in wheelchairs for boarding. I immediately stopped the boarding process and said we cannot board without the third Flight Attendant. He told me that there was a fourth jumpseater. I relayed to him that she was neither in uniform nor in a deadheading status on this flight. Obviously annoyed; he turned and left the two passengers in the jetway to wait for the next twenty minutes and became insistent that he would not take them back up the jetway (because they were so heavy). For the sake of not continuing to look like buffoons and for passenger comfort; I allowed the [wheelchair] passengers on board the aircraft knowing it was a violation of FAA/Company procedures. No other passengers were allowed on the aircraft until the last Flight Attendant boarded.