First of all, nearly all colleges require special permission to triple major (beyond the signature of his academic advisor). No college that I’m aware of allows a student to declare triple major immediately. He likely needs to convince quite a few people at his college before he can declare 3 majors. For his third major, he likely won’t get the consent to declare it until at least his junior year.
Then there’s the question of why triple major. There’s certainly synergy among the three majors, but that doesn’t mean it makes sense. As others have mentioned, no grad school or employer will give special consideration for triple major. There’re some disciplines of physics (e.g. theoretical physics) that require much more math (including abstract/pure math) than others. If he’s interested in one of those (he may be if he likes topology), he can certainly double major in both. There’re also some disciplines in CS that are much more pure/abstract math intensive (e.g. CS theories), some much more applied math intensive (e.g. deep learning) than other disciplines of CS. It may make sense to double major if he’s interested in one of those areas. If his interest is in quantum computing (especially on the hardware side), double majoring in CS and physics may make some sense (but is still unnecessary).
To summarize, I see some benefits of double majoring, but no benefit of triple majoring (and certainly no need to contemplate it right now).