Will the RC4 transponder with red,white and black wires work with a track running RC3 setup?

No. The regular rc4 (3 wire) will only work with the new decoder.

What nonsense for Mylaps to make a new transponder that is not backwards compatible with the "old" decoder. Maybe its greed more than nonsense. I was at larger race where the track did not have the new decoder and some racers showed up with the new new Mylaps transponder. All I kept hearing was the race anouncer say, "If you have a four wire Mylaps transponder, it won't work on this track. Remember to pick up a house transponder." Rediculous.

they should have made all the RC4's "hybrid" that would have made more sense. I have one local track running the new setup and one running the old setup

Yeah stormer's banner advertises "89.99 brand new mylaps rc4 transponder", when they don't mention it won't work with all the systems. Their banner even states it's not a hybrid and they have a photo right next to it of an rc4 2-wire hybrid. It's not stormer's fault, it's mylaps fault and their monopoly of the transponder market. Noone wants to switch decoders for something cheaper, thus forcing everyone to buy the new uber " $120 hybrid rc4". Sickening.

Original question asked and answered.

If people wish to complain and wrongfully blame Mylaps for not having any competition do it here.

But first question why MRT didn't do a better job of copying, cloning, duplicating, AMB/MYlaps signal in the first place? If they had all their PT's would still work, just like all the AMBrc PT's do...