You might have an extra one in the chamber as well so should we take that into account too or not? What if the rifle jams and you end up to another number not divisible by 3 again? What if the soldier fired a few rounds in semi auto before switching to burst?

Clearly the solution is that everyone should be given a few loose rounds so they can top their mags up to a number divisible by 3 whenever they get to an unacceptable situation where the number of rounds left in their mag is non-divisible by 3 during combat. This should remain their top priority. You can't acceptably allow the rifle to fire one or two rounds when you expect three. The enemy might file a complaint if that ever happened. Also pay no attention to the fact that the M16 already isn't guaranteed to fire three rounds on its first burst because the 3 round burst mechanism "counts" the shots in semi auto and it doesn't reset when you switch it to burst.

Clearly what we need to do is invent some sort of Quantum magazine that will always contain enough rounds to complete a burst no matter what happens until then. Burst mode is simply ahead of its time. The other technologies need to catch up.

Turns out that 25 is more than that number though...

-1?

So you use 25 bullets or a curved mag. Since the FAMAS was apparently designed with straight mags in mind (I assume they are cheaper to make and/or more durable). I guess soldiers with OCD can shoot the extra bullet semi auto after chambering it. Or maybe they load their mags with 1 less bullet and there's a guy running around with an extra mag filling it with these extra bullets /s.

In all seriousness though, I wonder if that has a part in the decision to move to the HK416. I would guess not, since I have never seen "real" criticism of the FAMAS on that basis (hell, I don't think I've ever seen someone have truly bad things to say about this rifle after using it, but I've never used one myself so what do I know). But I will try to investigate.