My Toilet Keeps Making Sounds Like It Is Filling Up Every 3 Minutes
Since you aren't seeing water all over the place, your flapper is leaking a small amount of water which is causing the float to drop to a level (apparently every 3 minutes) where it detects the water is low, so it fills the tank back up. If you aren't familiar with the mechanics of the float/flapper system in toilets, take the tank lid off and flush it a few times; it's pretty easy to see what it is doing. You might be able to slight adjust the flapper by just running your thumb around the part that seats or you might need to replace it (it's cheap and easy).

As far as electric bill, it won't make your electric bill higher if you live in the city since water is fed by gravity, but in the country where water is pumped from underground, it would. The bigger concern would be water cost if you are in the city.