A single cylinder engine will have one power stroke every two rotations.
At 3,000 RPM it will have 1500 power strokes.
A three-cylinder engine will have 3 × 1500 = 4500 power strokes per minute.
Where did you go wrong?
... we have power stroke each 240°, ...
OK so far.
... as we have 1500 rev. Of 720° in this rpm, ...
An odd way of looking at it, but OK....
A 1 cylinder 4 stroke at 3000 rpm will have 1500 power strokes,
So 1500 * 3 is 4500 power strokes.
Based on a 4 stroke engine needing two crank revolutions to complete the full cycle of intake, compression, power and exhaust.