It would be convenient, perhaps, but they have mentioned that as the rewards are geared for 3-6 characters, they are not planning on removing the limit (whatever that means is beyond me, and I don't remember/know if they were talking about Nyzul Isle Investigation or all the Assaults).

But yeah, they are doable with 3 characters, at least if you use the no level limit option when entering.
I recently finished the last one of them, and there certainly are some missions that can go bad easily even for level 95 characters, as there are many missions that really have nothing to do with character levels (excluding some movement speed enhancing gear).

Lebros Supplies for example is possibly the most annoying and horrible mission I have done, and while it is very likely possible to do with just 3, I would not recommend it unless you are curious/adventurous enough. ^^;