Learn English – What do call individuals who express their opinions as if they were facts

There's a CNN article about “smartphone zombies” that implies zombie is appropriate (perhaps in its sense “A person, usually undead, animated by unnatural forces ..., with no soul or will of his/her own”). The article also mentions Stephen King's novel, Cell:

... the protagonist, Clayton Riddell, doesn't turn into a zombie because he doesn't own a cell phone. The story is about an event called "The Pulse" that turns cell phone users into vicious, mindless beings.

You're right in your suspicion that "in threes and fives" is a literal translation and not idiomatic. You mention "in ones and twos," which is what I would have suggested as an answer. If you're looking for a phrase that means people coming or going specifically in groups of three to five, I can't think of one, but you could try:

The people were coming (going) in dribs and drabs. ("A series of negligible amounts.")

This gives a sense of disorganization, of a few here, a few there.

But in ones and twos is idiomatic and I would suspect quite satisfactory for most purposes: in small numbers.

Finally, it would be perfectly acceptable (though not idiomatic) to say

The people were coming (going) in groups of three to five.