When I play the game in singleplayer it works fine, it always crashes with this bug "cannot retrieve file ./DATA/RMTQ29SC.BIN I tried getting it from my friends who also downloaded this game from GOG, but it still doesn't work. When we play in game ranger it crashes, his game works fine though. I also don't understand why he has these folders that I don't. I have tried reinstalling the game with no luck and running it in admin

My brother and I had this same issue where my game wouldn't launch on game ranger for multiplayer, but would work for the campaign. I uninstalled both GOG and HOMM 3. Then deleted all folders associated with those two files. Then dug through my registery and searched for GOG and New World Computing and deleted everything with a GOG.exe and a HOMM3.exe. There were several .exe in deep with in other folders for some reason. Just be careful when doing this to make sure you do not remove any system operating files. After doing all this I reinstalled GOG and HOMM3 and it worked on Gameranger again. I hope this helps you get back to gaming with your friend!