so I was wondering on how to use several EQs at the same time

so in my setup I can tweak the eq in several places:

*my fuzz pedal has mid and tone control
* my multi FX pedal has EQ options aswell
* and finally the amp

so my question is : how do all those EQs add up ??

lets say I have the mids on my fx pedal at -3 and the mids on my amp at -3 aswell will I get -6 or will it stay at -3 ?

and lets say I set up the eq on my pedal and I set the EQ on the amp flat,
will it keep my pedal EQ settings or will it mess it all up?

other tips on EQ setup are welcome

thanks in advace