CAPT OF A FALCON 20F TURNED ONTO BASE LEG TO FOLLOW WHAT HE BELIEVED TO BE THE ACFT TWR INSTRUCTED TO FOLLOW ON FINAL. HE NOTICED A SECOND ACFT ON FINAL AND STARTED TURNING BACK TO THE DOWNWIND LEG WHEN THE TWR CTLR INSTRUCTED HIM TO ALSO TURN BACK TO DOWNWIND AND TO FOLLOW THAT ACFT ON FINAL.

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WE WERE COMPLETING THE VOR-A APCH; CIRCLE OVERHEAD FOR A L DOWNWIND FOR RWY 19. ON MIDFIELD DOWNWIND; THE TWR ASKED US TO SLOW SPD FOR TFC AHEAD. I SLOWED TO APPROX 140 KTS. WE RECEIVED NO OTHER INSTRUCTIONS FROM THE TWR CTLR. I SAW ANOTHER ACFT ON A 1 1/2 MI FINAL; AND TURNED ONTO THE BASE LEG RELATIVE TO THAT ACFT. UPON COMPLETION OF THE BASE TURN; I SPOTTED A SECOND ACFT ON FINAL BEHIND THE ONE I HAD ASSUMED WE WERE TO FOLLOW. AT THE SAME TIME; THE TWR INSTRUCTED US TO TURN BACK TO A DOWNWIND LEG AS THERE WAS ANOTHER ACFT WE WERE SEQUENCED BEHIND. I RETURNED TO A DOWNWIND LEG TO FOLLOW THE SECOND ACFT FOR LNDG. OBSERVATIONS: 1) THE TWR CTLR'S WORKLOAD WAS VERY HVY. 2) THE TWR CTLR SHOULD HAVE ADVISED US TO EXTEND THE DOWNWIND LEG; OR LET US KNOW THAT HE WOULD 'INSTRUCT US WHEN TO TURN TO BASE LEG.' 3) I SHOULD NOT HAVE ASSUMED THE FIRST ACFT WAS THE ONE WE WERE TO FOLLOW. I SHOULD HAVE ASKED FOR CLARIFICATION. 4) THE FO WAS BUSY COMPLETING THE FINAL LNDG CHKLIST; WHICH WAS OCCUPYING THE MAJORITY OF HIS ATTN AT THE TIME. 5) OUR POLICY IS TO MAXIMIZE 'SEE AND AVOID.' I NOTICED THE SECOND ACFT IN TIME TO AVOID A NEAR MISS; EVEN IF THE TWR HAD FAILED TO CATCH THE DEVELOPING PROB.