I have a 3 switch plate on my wall. One switch is for an indoor light switch ( 3 wire, as there are 2 on the same line). The other 2 switches are two separate outdoor light fixtures. I would like to install an in-wall timer that controls both outdoor light sets at the same time. Is this possible?
I don't quite understand your current arrangement but I can tell you that the timer needs to go in series with the source for all the lights, and between the source and only these light circuits.
This can get pretty tricky.
Once you have a wiring diagram (not the same as a schematic) for your existing setup the necessary changes will be obvious.
Getting this diagram will be difficult, even if your wires follow the color code.
Make drawings of your existing setups in each box so that you can return to the original configuration if necessary, because the worst outcome is that you end up with nothing working and breakers that trip instantly.
You'll need clip leads, a DVM, an extension cord, and a test load, e.g., an incand. bulb. A helper will make things go faster.
I recommend getting one set of lights working at a time. Since switches are cheaper than timers, I'd substitute a switch for the timer until you're sure everything works as planned.