Undefined, obviously.
Three is definitely a small group of people.
I would consider 20 to definitely be large enough to constitute a mob.
I guess I would consider the “mob line” to be crossed somewhere between 10 and 20 in most circumstances.
YMMV
In communications, a small group is generally understood to be 15 or less. Beyond that, direct communication is not regularly possible. I would susspec this number to be the lower limit of a mob mentality.