Mags are often designed to hold rounds in multiples of 5, so ten, twenty and twenty five can be found for some guns. 3 round burst is common, so a rifle having a 3 round burst and a magazine in multiples of 5 is super normal. Many US military automatic rifles like the M4 also fire 3 round bursts. 20, 30, and 40 round magazines are available for the M4. French ain’t got nothing to do with it having non-multiples of 3 in all magazine sizes. Also, the FAMAS has an available 30 round magazine…which is divisible by 3.