for years i;ve been running the fuel pump on a 3amp fuse.

never blown. polo 6n. is it okay to carry on using the 3amp.?

You've answered your own question if it hasn't blown up till now why should it start, unless you've changed the pump to a different type 

no , same pump, just using with bike carbs.

 

You could connect an ammeter in series and measure the current when running, or measure the pump resistance and calculate max current at 14.4volts.

Then add 50% safety margin for initial inrush current. 

 

I think it was Jaguar E-type where the electric development involved fitting bigger and bigger fuses until they stopped blowing.

 

3Amps x 12volts is 36Watts and that sounds like plenty of electric motor power to me.

Remember the old saying, "If it ain't broke, don't fix it"

 

Regards