yes a 3" will take both 2 3/4" and 3" shells.
the reason i specified to buy a barrel with a 3" chamber, is because there are people throwing old 2 3/4" wingmaster barrels on the EE. id pay up to $80 for one, if i wanted one for a cut-down project, but for the most part theyre useless when for a little bit more $ you can buy a 3" barrel. the old Wingmaster barrels also came with fixed chokes.
even if your gun is only capable of 2 3/4" (incidentally, 870 receivers are identical, the old 2 3/4" guns can be upgraded to 3" capability by swapping in a new ejector blade and ejector spring), its still smarter to buy 3" aftermarket barrels both for better resale value, and in case someone slips a 3" shell in your gun by accident.