Depends on how much you get charged from your company. Look on your power bill see how much a kilowatt is times that by 3 and then times that by 24 bingo...
Three 1000w fixtures will use 3kw/hr for each hour the lights are on. So 3x12=36kw/day for 12 hrs of lighting during the flowering period. 3x18=54kw/day for the pre-flowering stage. At this usage you would use 36kw x 30 days or 1080 kw/ month during flowering and 54kw x 30 days = 1620kw/month. If your rates are $.10/kw hour it costs $160/month for the pre-flowering stage and it will cost $108/month in the flowering stage. Bear in mind this is only the usage for the light fixtures, there will be additional costs involved to run the fans and pumps for this grow and as stated previously some electric companies have tiered rates that increase with usage. Also remember this is only for the energy for the grow room; you have to add this to your current bill to get the total monthly bill.
here in good old socal, they rape us to the tune of 34 cents per kw/hr in the third tier of the billing. I can easily hit the third tier even with no grow wattage, so when I grow, pretty much every kw is third tier price. $357 and some change to run 3 1000k @ 12hrs per day for the month.