3mm screws are hard to find at hardward store. the smallest I've
seen are 4mm at home depot or home base in those draws.

Hobbie shops usually have them, most .40 size engines use 3mm to attach
the muffler.
Dubro , with verious length is what my LH carries.
Don't forget to get an allen wrench, I belive they are metric also.
Use blue loctite or run a little bit richer to prevent lossing your muffler in flight.

Sometime that nut at the end of the muffler gets loose and the back end
of it gets lost.
you might also want to get some 3mm nuts to double up, so it won't get loose.

I've lost enough of them, as many other had also.
I've found many mufflers and what not at the air plane's grave yard right
beyound the flight line.lol