3 is def not overkill. In fact, running 2 is no where near enough. At 50 sq/ft and 1200 watts, you are only lighting at 24 watts per sq/ft which is only half of the suggested minimum 50 per sq/ft.
That said, are using every square inch of the tent? If you are not and the plants are lumped together around each light then that's ok.
For a 5 foot wide tent, 600's are not enough. You should be running two 1000's.
Three 600's will adequately give you coverage across the 10 feet but not the 5 foot width.
Regrding the heat, adding a 3rd light is going to increase the amount of heat being generated by 33%. If you are running sealed hoods and exhausting the heat then you can deal with this. You might only need to increase the strength of your fan(s).
