How fast target is moving in both left and right and toward and away? As well as how big is the target? How much space to mount your transmit pinger on the target?
Concerns on 45 foot distance with ultrasonic.
Correct. Read more how HC-SR04 works, on-board uCPU, that generates a pulse with a length proportional to distance, starts the sequence only after you trigger "send" pin. So , it sends a pulse and wait a response. In your model with 3 units, one as a transmitter and two as receivers, you would need some way to synchronize 3 units, because receivers also needs "start" events (even it doesn't send anything - transmitting piezo removed) in order to switch in "listening" mode. You may use cheep 315 - 434 MHz radio for this. It would be still possible to build an "asynchronous " system, when two receivers get same "trigger" events periodically, not necessarily the same time with transmitter. For example, transmitter pings each 1 sec a pulse. You trigger receivers also every second, but doing so signal may arrive immediately or at the end of period. It's not important, if you don't need to know a distance, just an angle. Than you get just a difference in between arriving Now, this guy in a link you posted, build two receivers on himself, most likely he is also did just analog part w/o special uCPU for each receiver. Your options - modify cheep HC-SR04 or solder your own. This uCPU, that build-into HC-SR04, does some noise measurements and keep adjustment of the comparator , so if you are not really into analog design, better to modify cheep unit. I'd suggest, start to experiment, have a scope?