You are not required to have three powers. You are also not limited to only having three powers. That section on the sheet is just laid out like that to let you know that you can have multiples. So feel free to create what you envision in your head:
That is specifically for characters with multiple powers to be listed separately. For instance a character with Fire Manipulation, doesn't need to list: Fire Generation, Fireballs, Flamebody in different lines, but should include all of that in the same general power... Making me aware of what they're Fire Manipulation is capable of.
If they have, say, Fire Manipulation and Super Strength, that'd be two different lines.