How many ways can 3 people stand in a line?

Six

If only the order is interesting (not the direction that they are oriented) then In the first place there can be any of the three, in the second place there can be either of the two left and then the last place is a given so: 3 * 2 * 1 = 6

Six different orders for three people standing in a line.

If their direction is also interesting to take into account (e.g. the middle person facing north while the others face east) then you have to know how many different directions and if standing on their heads counts as different etc.