Defend the Border (quest) reads: While (this card) has 3 or more resource tokens on it, redirect the 1st point of damage done to your capital each turn to another target unit or capital.

Do I "spend" 3 resource tokens to trigger this effect, or do the resource tokens just keep piling up as long as a unit is questing there, without ever being returned to the token pool?

Thanks!

They just keep piling on and are only removed if the unit is removed from the quest. If you're playing with just the core set, there is no need to keep adding tokens after the third, since there is no way of removing just some of them.