Why can't I put a 13A fuse in instead of a 3A?

solder a nail in it

Tin foil

It'll work, but won't protect you against certain overload conditions.

All very confuseing

I'm getting amped here.

Confuseous he say - ALWAYS use the correct phews

Confucious say man who walk through airport terminal naked going to Bangkok

can you scratch of the "1" with a sharp knife? hey presto, a 3A.

...or just write one on if it's the other way around?

Need to paint it red as well.

COMMUNIST
REPORTED

Göring ,
Assuming your not being obtuse (get me), the fuse needs to be the weakest point in the circuit, if you up it from 3A to 13A the cable maybe unable to carry 13A so in the event of a fault it will overheat and the insulation may set on fire. Current is the equivalent of the amount of water flowing down the pipe and voltage the pressure.
HTH

You can providing your body can take 12 amps

Burn, baby, burn!

Bentley Boy is correct. 13A could be used as a very temporary expedient, but 3A fuse should be substituted ASAP.