I'd like to use MC34063A as step down converter from 9 V to 3V
my question is , how can I determine the output current? is there a formula for that? can somebody tell me
The load that is fed by the 3 volt supply will determine the current. If it is resistive, then Ohm's law works - I = E/R. Current equals the voltage divided by the resistance.
If the load is active, the spec sheet for that device should give you the information.