Why can you only load 3 shells into the mag and 1 in the chamber of your Browning A5 12 gauge?

It depends. Has the magazine plug been removed? Remove the forearm and the barrel. The magazine may have a wood or plastic plug in it. If the plug has not been cut, then only two standard size shells should fit, along with one in the chamber; This would limit the gun to only three shots as allowed for waterfowl hunting. With the plug removed, the gun should handle four shells in the magazine and one in the chamber. Magnum model receivers have extra room if using only 2/3/4 shells. If the magazine tube has no plug and you can load only three shells, check the spring and possibly replace if needed. There is one other possibility and that is if you have a three shot model made prior to the 1950's. Only the original forearm will indicate this, in which you should only be able to load two in the magazine and possibly have a bad spring as well.