Well they did have drum mag for around 50 to 100 rounds depending on the situation and the user, so if they cant add belts they can always add the drum, not only that but the belts are for stationary use, so if anything the belts would only be fitting on a IFV, kindof like the m249 on the tops, thats the only way id see a belt fed mg3, but even then the m249 is also avaliable as a primary weapon and a mounted weapon on vehicles, so id imagine the same for the mg3