Leaving Certificate Mathematics Question 13 by Maths Grinds

A student is testing a new homemade plant food. He plants three bulbs in three different pots and then counts how many germinate (grow) after a month. Denote a successful germination as S and an unsuccessful one as U. The student considers the sample space (the set of all outcomes) to be:

{SSS, SSU, SUU, UUU}

But this is incorrect. Why?