Helicopter pilot reported a near miss during lift off with another helicopter on approach on the same helipad. Both pilots took evasive action to avoid a collision.

Narrative

Had a near miss with a helicopter on RTB leg. At the time of the occurrence; there were four separate helicopters airborne and in close proximity in the downtown Philadelphia airspace. This airspace is Class G and is where all the main hospitals in the Philadelphia area are clustered. Air-to-air Communications happen on one frequency. I was lifting from a helipad; and Aircraft Y was orbiting on the North side of the city waiting to land at the same helipad 1. I opted to track on the South side of the city to avoid this incoming helicopter. At the same time I was lifting; there was a call from Aircraft Z; who was 'South of the stadiums and inbound to helipad 2.' Shortly after that; Aircraft Y called 'final for helipad 3.' Then Aircraft Z called final for helipad 2. At this point; I had thought I had visually acquired Aircraft Z; and made the call 'Aircraft Z; Aircraft X has you in sight; I will be passing in front of you from left to right; 800 eastbound.' A couple seconds later a saw an aircraft appear within 100 feet at the same altitude heading on a collision course. Both myself and the other aircraft made an evasive maneuver. Just after the near miss; the Aircraft Z aircraft pilot said on the radio 'what was that Aircraft X? ; you said your were passing behind me?' I replied; I misidentified you as another aircraft; I didn't see you until the last second.' Aircraft Z replied 'That was almost really nasty; brother.' To which I replied 'Indeed it was.' In hindsight; I believe that the aircraft I had visually acquired was actually Aircraft Y; who was on final approach to Helipad 3. I had never actually visually acquired Aircraft Z. However; my radio call warning to them was correct; and misunderstood. I stated that I would pass in front of them; not behind them. Further informaton: I do not remember seeing their landing light; but my anti-collision; position lights; front crosstube landing light and strobes were all on. This incident happened at night; and my goggles were flipped up due to the magnitude of extremely bright lights; and cranes with LEDs; in downtown Philly. Of significance: Our TCAS was inop and on MEL deferral. I usually always fly with the TCAS screen up; but it was of no use. The crew and I pulled flight tracks from ADS-B data and determined that this incident happened over 7th and South Streets over downtown Philadelphia. We also debriefed with ; who pulled the tapes for the frequency and we listened/replayed all the radio transmissions leading up to this event. This was the closest call i have ever had in my XX years of flying helicopters. Myself and my crew where shaken. We opted to use a safety time-out; and not take anymore flights for the remainder of the shift.