I'm sure that others will weigh in here, but here are my two cents.
I would say it depends on their willingness to re-balance and otherwise maintain their 3-fund. If they will be contributing more in the future, then they will have to re-examine the proportions of each fund with each purchase. To some folks this is no big deal (I think it's fun!), to others it's a pain in the ass. If they are of the latter, then a single fund might be the best choice for them.
Since they are young, another approach may be the two-fund approach (total domestic + total international), or even a Target Date fund. These might give them a little more diversity.
Cheers.