Was at SEA 4hrs before my flight yesterday and was reminded of the 3hr rule when trying to access the Centurion lounge, which didn't look too busy at the time. The front desk attendent suggested I try the Delta lounge and come back in an hour. I reminded him of the irony of not being able to use an Amex product at a Centurion lounge but being able to use it at a 3rd party lounge instead. Would have been nice for him to exercise some discression, especially since the lounge wasn't full.