If you have three, you have three. If you have two, you have two. If you have one, you have none. What is it?
This is kind of a play on words, but:
If you have 3 options, you have 3 options. If you have 2 options you have 2 options. If you have only 1 option, you don't have any options.
Along similar lines, this works too:
If you have 3 choices, you have 3 choices. If you have 2 choices you have 2 choices. If you have only 1 choice, you don't have a choice.
If you say the riddle aloud, the answer could be:
"one" sounds like "won". If you have 2 or 3 competitors, then you have 2 or 3 competitors. But if you have "won", then you have none.
Stack Exchange Reputation Score?
If you have three, you have three.
If you have two, you have two,
but if you have one, you have none.
as it is the default rep score here. If you have 1 rep score you really have none.
It may be
the number of consecutive consonants at the beginning of the word for the numeral.
THREE ↑↑↑ = 3 (three) TWO ↑↑ = 2 (two) ONE - = 0 (zero, i.e., none)
And so it turns out that, if you have none (or zero), you have one — which somehow seems appropriate.
It probably isn't, but could be:
Neighboring cells in Conway's Game of Life
A cell with two or three live neighbors will be alive in the next iteration, while a cell with only one live neighbor will die.
It is a
In fact those are
Three repetitions, repetitions, repetitions.
Two repetitions, repetitions.
A Guinea Pig. Several Guinea Pigs will thrive, whereas a single guinea pig will become lonely, and soon die.
Is it a
If you have triplets, you have three multiples. If you have twins, you have two multiples. But if you have a single child, you have no multiples.
Original answer, not correct by the precise definition (thanks @Will):
If you have three organisms with the same DNA, you have three clones. But if you have one organism it is not a clone.
It is a...
Collection of things
If you have three, you have three things, if you have two, you have two things, but if you have just one, you just own a thing, and it's not really a collection at all.
It may be about physical sharing, and telling other people what you have.
If you have more than one unit of something you can inform your surroundings on this and share it with them.
On the other hand: if you have only one, telling others might induce envy and you may be better off not mentioning it.
Thinking visually, it could be
corners in the numerals 1, 2, 3 when drawn by hand. 3 has the upper right, middle left, and lower right; 2 has upper right and lower left; but 1 is usually a straight line.
The solution is
You don't offer the last one.