It get complicated, usually the three hour rule is if they send you home because there is a snow storm or some such event that makes it impossible to work your shift.
So they pay you for three hours of your shift.
If they ask you to go home then they pay you for what you worked.
Now you have the right to to stay for three hours.
If you want to be cranky, you can say I would like to work out my three hours because that is a base shift.
It just won't make you popular, not that standing up for yourself ever does at a place like Spot.