Power is the rate of doing work. i.e. it is the amount of energy consumed per unit time.
Power = work/time
Now, work = 3600 J and time = 3 seconds
= 3600 J/3sec
= 1200 Watt
because 1 watt = 1J/sec
Thus, the power when 3600 J is done in 3 secs is 1200W