First, note that existence of anything is not claimed. If some world is completely devoid of potato heads (as a result of war or pestilence...), then this statement is true in that world. The statement claims that the existence of three or more potato heads is impossible.there is at most 2 potato heads
Therefore, an equivalent, but, perhaps, less intuitive, was to say this is: It is not the case that there exist three p.h.'s and all of them are different. Note the last part because the formula is true in a world with just one potato head.
For the second statement, as a hint, look at the definition of the uniqueness quantifier.