Hmm Mikaila makes a good point, in 5-10g setups, airpump filters would work well and you can run 3 off one pump, so its like consuming the power of 1 and the heaters would run longer since they need to heat the same volume of water anyway.
However, in 3 10g tanks heat loss is not terrible your heaters may not actually have to run as hard, for example, using a jager heater, the smallest wattage is 50w, for 10-30g aquariums. In 3 tanks, your heat source would be 3x 50w heaters. However, in 3 10g setups, you would only get 2 50w heaters. The flow of water should even out the temprature differences. You might even be able to get away with only 1 50w heater. Although 3x heaters would run for less times and consume less power, whereas 2 heaters will run for longer, the overall power used will still be less since all heaters still use power even when not actively heating. The actually amount of power saved is probably negligible, but anything short of hooking a system up to wind power or something, more tanks will always take more power.