I have an exxisting circuit with two exterir light. I want to add a third light but also put the lighting on a photocell. I ca not get access to the wire between the swith and the first light . How can i install the photocell?

Most photocells have a neutral, a black(line in), and a red(line out). You need to get between the lights or have multiple photocells.

You can put them on each light, they aren't much. Do not connect them with the breaker on, you can screw them up wiring them live... you shouldn't wire anything live anyway but some people think they are better insulated than the rest of us and still do it for some reason.. lol...

I hate to say it, I'm one of the ones who works things live , I'm not more insulated, but my shoes are, I just have to be aware of tacks in my shoes.
I have changed many electrical/electronics Live, Just make a quick, deliberate, and exact connection. During an Emergency, I have changed out a Meter Can Live, Alarm Keypads. I have a Leatherman I carry all the time, All metal, when changing out a receptacle with ground attached, remove ground first and connect ground last when dealing with recepticles.
I don't recommend anyone doing this without understanding the risk, death.
I have done it so long live it is a non event, like when a bird lands on a high voltage line, he is not thinking of the danger, but in his case, he is not aware of the danger.
I have yet to destroy something by connecting while live, remove the load if possible.

To be safe, turn off power.