I consider 1 good light to be >95% (almost completely arbitrary, yes) of the way "there" as far as active rear lighting goes. Anything more has drastically diminishing returns and there is no 100%. I usually run 2 with a 3rd for the helmet on certain routes. Lately, I've just been moving light #2 from my seatpost to my helmet on these routes.

If the chainstay lights are to be angled out a bit to the sides I understand but, if everything is to aimed straight back, you could find a better spot higher up (ie helmet) for a 3rd light, IMO. Make sure that the lights and brackets play will play nicely with the the angle of the chainstays.

I did stuff like this for a while, but it's a PITA when getting prepared for a ride or locking up in public. Juggling 3 taillights plus headlights and helmets, gloves, locks, making sure batteries are charged etc. Not sure of this is an issue for you but that's where it became overkill for me.