You may want to look at the rate structure too. The three phase service will likely be a commercial rate and could also include a demand charge. this could make your residential use actually more expensive than a seperate single phase service. Also consider voltage drop loss from your shop to the house and the maintainence cost if it is your wiring.
Also, if you deduct the shop power as a business expense you will have problems with the IRS if the house consumption is not metered by a real electric meter. I did it the other way around and fed my shop from the house service through my own sub meter. I read the meters every Jan 1 and deducted the percentage of use for the shop from my household expense.