Ah, it's apparently 90% have two tanks. The remaining 10% have 3 and have pre-mixed, because they had leftover tank from when they did leaded gas, and by doing

Ah, it's apparently 90% have two tanks. The remaining 10% have 3 and have pre-mixed, because they had leftover tank from when they did leaded gas, and by doing this they don't need to upgrade the pumps. So it's probably old/cheap/un-upgraded stations which have 3 tanks.