Use the formula: [tex]P=IV[/tex], where P is power (watts), I is current (amperes) and V is potential difference (volts).
For the first heater:
[tex]900W = I_{1}*120V[/tex]
[tex]I_{1} = 7.5A[/tex]
For the second heater:
[tex]1800W = I_{2}*120V[/tex]
[tex]I_{2} = 15A[/tex]
For the third heater:
[tex]3000W = I_{3}*240V[/tex]
[tex]I_{3} = 12.5A[/tex]