1) if it is 4 wire counting ground(3 wire plus ground) and the 2 hots are on opposite legs, then yes you can. if they ar eon teh same leg, then no you can't. AC current is additive when in phase and cancels out when out of phase. so if they share the same leg you could potentially put 30A on that neutral and smoke the wire. but opposite legs it can never be more than 15A on teh leg.

2) no clue not a pro.

3) GFCI can protect all down stream devices. i THINK as long as it is only other baths on teh circuit it is legal. i know that you can't share the bath circuit with other rooms that are not a bathroom, just not sure if teh 1 can feed 3 baths or not.

1. Look in the breaker box and if the wires are on a double pole breaker you are fine. Hot lines alternate on breakers so a double breaker hits both hot lines.

3. Everything in one bathroom, (lights, outlets, fan), on a breaker or outlets only in all bathrooms, your choice.

"1) can you run 3 wire(with a ground) in a residential building to two separate circuits? EXAMPLE: Our dishwasher, and disposal are each on their own circuits, but share a common neutral..? Isnt that an unbalanced load?"

Not a problem. It's called a multiwire branch circuit (MWBC) and, as D&K notes, the two hots have to be on opposite poles. The neutral only carries the imbalance of the currents on the opposite poles. Also, to be fully compliant with current code, an MWBC should use a common (i.e., two-pole) breaker. Previously, a handle tie was permitted and, before that, two independent breakers were allowed.

"2) Can an arc fault breaker installed in bedrooms, also be hooked to lights? Both of my kids bedrooms share a 15 amp arc fault and their lights are on it too."

Yes, this is legal.

"3) Our bathrooms(all three of them), share a 20 amp circuit(the outlets), the first one has a GFI, but if you "test" it, all of them quit working, so it is a shared line."

If newly installed, it wouldn't meet code to have three bathrooms on the same 20-amp circuit. However, your situation may be 'grandfathered', depending upon when the wiring was done. It's more a matter of capacity than imminent risk. That is, you'll probably trip a breaker if two hair dryers are used in two bathrooms simultaneously, etc.

the wiring was dome about 3 months ago...brand new house!

Yes it is all to code, just not to my company's personal code :) Any number of bath receptacles can share one 20 amp circuit as long as the bath lights are not included. The shared neutral is fine as long as separate poles are used. Does not even need a double or tied breaker unless both circuits are on the same receptacle or if they use 2008 code which is doubtful. You could put every bedroom, lights and outlets all on one 15 amp circuit if you really wanted to.

I try to always have a separate circuit to each bath so 2 hair dryers can be used. If the customer is being a cheap a*^ and bringing in dumbo electric to bid against me, sometimes I'll keep it to code only and circuit the baths together. I always try to do it the better way instead after the customer knows it costs a bit more to be more than minimum code.

Funny what the code allows sometimes, but it is minimum

"Does not even need a double or tied breaker unless both circuits are on the same receptacle or if they use 2008 code which is doubtful."

Nods. According to this link, Kentucky adopted the 2008 version of the NEC on 7/29 and began enforcing it on September 1. So, if you've been in the house about three weeks, it might have slipped in just under the wire and the 2005 code may have been in effect at the time of the final inspection. Looks like it's right on the bubble.

I looked at the final inspection for occupancy, it was signed 8/25/09, they got me by 6 days... Bastards! I am amazed what "code" allows them to do... When we built our house, standard was 200 amp service with a heat pump, we upgraded to 400 amp... they said I am the only person to ever do that... I never assumed they would screw us with such few circuits, it's amazing how much is daisy chained. Both of my girls bedrooms are on a shared 15 amp arc fault, our bathrooms share a common 20 amp cicuit for the outlets. The kitchen got 2 circuits only for the appliances. The disposal and dishwasher got their own, but 15 amp... Microwave and fridge are each on their own with 20 amps. I really figured a home builder would do a better job. Dont buy a Ball Home...

if you are an electrician as you claim why didn't you check all this out while you were looking the house over and doing walk throughs? That's almost like buying a car without even driving it.

Second, all this information can be easily found in the NEC. If you are actually an electrician you should at least be able to know how to look something up in the NEC regardless if you work in residential or not.

One I was in the other day built in the last year managed to do the 1980's style GFCI looping but still to code. Basement GFCI heads up to the garage receptacle, then on to 3 different outdoor receptacles. Trim out the whole house with 4 GFCIs. Counter 1, Counter 2, All bathrooms, (Basement, garage, outdoor). Makes Christmas lights, saw in the garage, and a shop vac in the basement run all together very well.

I also like multi million dollar houses wired with all receptacles on 14/2 using runs that are well near 200 ft long. On top of that, a circuit full of receptacles gets the entry chandelier loaded with 940 watts of bulbs, and the only outdoor outlet thrown on there as well. Big real estate agent we do work for bought a house like that and had me to the Christmas party. Of course I got to figure out where to run extension cords 10 minutes till the party started, trying to light the roof full of Christmas lights which could not run with the entry way lights turned on.

Maybe the code needs articles for dwellings valuing $200k + Sure would help keep the bidding consistent ;)

The code is only concerned with safety, not convenience.