i seem to recall there maybe being a button in the M3 simply to boot slo 2 games. but i dont have one so could be wrong. however with my flashed ds and M3 (slot 2) if i hold down select while turning on my ds it will boot to the ds menu automatically and then i sleect boot gba game. try it
Well, the M3 simply does have a second slot boot button which doesnt work with any of firmware's, the select button trick doesnt work as well. Thanks for trying to help.I made some research and i found the solution, u need a file that specially boots the gba slot, and it's as simple as starting any ds rom, if anyone having the same issue as me, here's the link, is a small file not even 1mb, it sends u to a green screen, there you just select start game and it boots as it should.
GBA boot tool