NMAC BTWN AN ACR MLG AND 2 ROTARY WINGED ACFT IN A XING TFC SITUATION.

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HOT; HAZY AFTERNOON IN WASHINGTON (HAZE DID RESTRICT VISIBILITY) FLYING THE RIVER VISUAL APCH TO RWY 18 AT DCA; CREW IS DCA BASED SO WE ARE VERY FAMILIAR WITH THIS APCH. AT ABOUT 4-5 DME; TWR ASKED US IF WE COULD LAND ON RWY 15; I DECLINED AND SAID THAT I'D LIKE TO CONTINUE FOR RWY 18. TWR THEN ADVISED US THERE WOULD BE 2 HELIS COMING OUT OF THE TIDAL BASIN AREA. (IT IS NOT AT ALL UNCOMMON TO SEE HELIS XING OUR FLT PATH WHEN WE ARE TAKING OFF TO THE N AND CLBING). I COULD SEE A CONTACT ON OUR TCASII SCREEN -- JUST ONE TARGET -- AND BTWN 600 AND 500 FT ON FINAL; WE GOT AN RA THAT COMMANDED CLB. I HAD THE FO LEVEL OFF AT 500 FT; THE TARGET SHOWED 200 FT BELOW ON THE TCASII AND VERY CLOSE ABEAM ME (ON 5 NM SCALE) AND I NEVER DID SEE EITHER HELI. WE STARTED BACK DOWN WHEN THE TCASII SHOWED WE WERE PASSED THEM; WE WERE HIGH AS WE ROLLED WINGS LEVEL ON FINAL BUT; WE WERE STILL ABLE TO SAFELY COMPLETE THE APCH. I HAVE BEEN FLYING THIS APCH AT DCA FOR 7 1/2 YRS; AND THIS ONE REALLY GOT MY ATTN. TWR IS NOT CTLING THE HELIS; I CAN'T BE SURE THE HELIS HAVE ME IN SIGHT; I KNOW THE SECOND HELI IS LOOKING AT HIS LEAD -- THEREFORE; I KNOW HE IS NOT LOOKING AT ME; AND I AM DSNDING INTO HIM WHILE ON FINAL ON A CHALLENGING APCH. THIS HAD THE POTENTIAL TO BE A MIDAIR; AND UNFORTUNATELY; PROBABLY IS CONSIDERED ROUTINE OPS BY DCA TWR.