3 wires will give you 2 hots and 1 neutral.
Not on a 240VAC circuit !!!!! That only works when supplying 2 120VAC circuits.
The neutral provides the return path for both 120VAC circuits. That works because each of them is out of phase and their currents cancel. If both circuits are pulling full load at the same power factor, the current in the neutral is zero, thus 1 conductor can handle both the return currents.
4 wires will add a dedicated ground.
No.
Each 240VAC circuit is going to require a hot (red) and a hot (black). There is no neutral in a 240VAC circuit. The only place neutral is provided in a 240VAC circuit is when the appliance (dryer or oven are the only I've seen) also needs 120VAC, in which case a neutral is also supplied, thus resulting in a 4 prong plug (hot, hot, neutral and ground).
BTW, it isn't called a ground wire anymore. Its called a "bonding" wire. The ground wire only exists at the main breaker box. Everywhere else is "bonded" to the ground wire.
I'm not sure how your electrician is going to set up 2 240VAC plugs. I suspect he is going to run a separate 10/2 to each and have separate breakers for each. I doubt you could run 10/2 (8/2 ?) to the first box and then split off of that to the second box.
I'd like to know how you guys are getting cables pulled into the kitchen (?) for these plugs in a finished house. Hopefully your basements aren't finished because that makes things a lot easier.
If you want to save yourself money, and you know what you are doing, ask the trade if you can pull the wire for him. That is usually 50% or more of the work on these types of jobs.
The other hint is to call the Electrical Inspector and ask him how to do it. Actually, they won't tell you directly. Tell him how you intend to do it and he will tell you if it meets code or not. Then he will tell you how to do it properly.
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