The "3-minute rule" at auction refers to a bid-extension (or "soft close") on timed online auctions: if someone places a bid in the final minutes before a lot is due to end, the clock automatically resets to give other bidders a chance to respond. Three minutes is simply one platform's chosen window — others use one, two or five.
The soft close kills sniping. It doesn't reward the quickest click — it rewards the buyer who set the right number and stuck to it.