I am wondering if it is possible to wire a single photo control to control three fixtures vs. each fixture using a separate control as is now. The circuit wiring goes like this:

Wall switch - Fixture 1 - Fixture 2 - Fixture 3
                                            |
                                       Wall switch 2

So, the three way switches have two fixtures between them and the third fixture is wired off the second fixture. I’d like to have the existing photocell in fixture 2 (or 3) control the whole lot (Fixture 1 doesn't get as much sunlight) and still have the two switches functional, but would be willing to eliminate one of the switches (and replace it with a single pole one) if needed.

This is 120V in the USA. The idea is to keep Fixture 1 off on cloudy days, reduce energy consumption (since each individual photocontrol uses some energy when the light is off) without having to add a junction box and external photocontrol at the beginning of the circuit like normally would be done. I'm not sure this can even be done, but it was an idea I had.

Maybe it would be simpler to replace one of the 3 way switches with a 3 way astronomical timer ?

Yeah I thought about that too. But the problem with timers is they don’t take into account the odd stormy day where it gets dark super early or gets light really late. I might just end up going thru photocontrols until I find one more sensitive that doesn’t turn on so early when used in a dark location. Unfortunately it’s a real pain to test and swap out photocontrols unless they are twist locks.

Understood it correctly you want to use only the existing wires, or there is a possibility to add few extra into the conduits?
And where enters the power feed?

What I did not understand is, how you want the controls to work.
Do you want the photocell to be the "master switch", which switches everything just off during the day (regardless of the switch position) and at night you want "2" and "3" always on and the "1" controlled by the switch(-es)?

And with only one switch remaining, could the remaining switch to be the "switch 2"?

Your options depend on how many wires are available between the boxes (of lights 1 and 2 and the switches), from where the supply is coming, and whether you need a permanent phase in any of the locations (for powering something else on the same circuit)