12Vdc to 5Vdc @ 3A circuit
Q3 needs to be turned on to provide current to the load. To turn it on you need to draw base current via the emitter and from the positive rail (in red). I see no conduction path that can achieve this; everything that connects to the base ultimately connects to the positive rail and hence there can never be any base current and there can never be any output voltage.

There may be other problems too but this seems to be the most fundamental. To be honest, the design looks a mess.

If you want a simple and effective 5V 3A design try this questioner's circuit that was posted an hour or two ago.