They must [do things] before they can understand these concepts.
This one is probably that which says precisely what you mean without either excess or ambiguity.
They must [do things] before they can begin to understand these concepts.
The begin provides emphasis, but seems a bit needless. I'd use it if I meant it literally. Conversely if I really did want to pile on emphasis (e.g. I was responding to someone who disagreed with the need for [do things]) I'd go all out and use "can even begin to".
They must [do things] before they understand these concepts.
Ambiguous with the idea that the time when they understand the concepts acts as a deadline by which time they must [do things]. It's not a very readily available misreading, so I don't think this ambiguity is a major problem, but it's enough to make me lean against it.
are any of them better suited to formal writing than the others?
I'd favour the second in any register.
My "can even begin to" alternative (again, only suitable if I've a strong reason for such emphasis) I'd be less likely to use the more formal I wanted to be; not ruling it out in any register, but the more formally I was writing the more I'd only do so when opposed to another opinion.