First, why would you run all 3 at the same time? Also, do you have a hot water heater large enough to support running all 3? Even an average gas hot water heater will run out of hot water shortly with 3 shower heads running.
Do you know how close you have 3/4" lines?

Good questions - It is a two-person shower and DH and I often are in the shower at the same time in the morning. He will have the fixed shower head. I could see times when I would want to have the rainhead and handheld on at my end. I agree that its limited by the available hot water in the tank. As for the 3/4" lines, we have the shower demo'd and it's all 1/2 in there, so we are guessing that most if not all of the plumbing is 1/2".
Thanks

If the plan is to have 2 people in the shower then you should have 2 control valves to adjust the temperature independly. The handheld will be turned off and on from one of the valves. It would be nice to pipe the shower from a 3/4" line but it doesn't sound like it is possible.

I highly encourage you plumb your shower so you can run anywhere from one to all three heads at the same time.

Figure out the two heads that will be used most often...for example, you using the handheld and your hubby the fixed. I'd have one valve control the handheld for you, the second valve control the regular shower head for your husband.

Then put a diverter on either valve to control the rainhead.

Or, get one of the A/B/C diverters that can send water from a single source to either A, B, or C, or any combination of the three. Here's an example.

It really will help with flow control.

If your supply is 1/2", then just use a half-inch valve. Do realize that with some of the diverter valves it's assumed that there is going to be significant flow required, so they might not be available in a half-inch size. Only larger.

Pressure balanced valves feed rainheads just fine. I've neither seen nor heard of a problem.

Before doing any plumbing, I'd see what flow volume you're getting out of the half-inch pipe, you'll need to know if it can supply three outlets at once.

Example, your three heads are each 2.5gpm, you'll need 7.5gpm out of that pipe to run all three at once. If it can, then you can do as you want. If it can't, then look into a 3x diverter. Or multiple valves, one for each head. Or something in between. Your choice.

If you go with a single valve, be aware that the valve may have a volume pass-through restriction. Make sure the valve can pass through the 7.5 gpm that would be required in the example above.

Mongo