There is a quest 'Defend The Board', which reads something like 'each turn if there is unit questing on this quest, place 1 resource token on it'.

What confuses me is that to trigger the effect on this quest card, you only need 3 or more tokens on it, so what's the point that after 3 turns you still put token on it? From what I played tonight, this quest card kept 'eating' resource tokens from resource pool...

Can anyone explain to me why it's like this? Did I miss something here?

When you are told to place resource tokens on quests or other cards, these tokens come out of the general supply, not from your personal pool you collect during your kingdom phase. So it doesn't cost you anything to keep putting them on the quest.

Right now, there really isn't much point placing more tokens on Defend the Border once it has three, but there are cards that allow you to add extra resource tokens to quests or other cards that can have them, and if we get one that allows to remove tokens, you'll want to make sure you have the correct amount on the quest.

Keep in mind that if the unit on the quest is removed from play or moved to a different zone, all the tokens are removed from the quest.

Alright, now crystal clear. Thank you for your answer.