In AlMg3, magnesium atoms outnumber aluminum atoms by a factor of three. In terms of mass, the ratio of aluminium to magnesium is a mere $27.0 / (3\times24.3) = 0.37$.
Shouldn't AlMg3 be better called magnesium alloy?
Class of product
Al-Mg alloy for MIG/TIG welding.
DIN 1732, SG-AlMg3, AWS A5.10, ER 5754
Nominal composition (weight %)
Al: Bal. Si: 0.4 Mg: 3 Mn: 0.5
Thus, the 3 in the product name refers to 3 % (in weight) of magnesium.