I've done 5 minutes with no issue, depends on the health of the fish though and why you are dipping.
It can take five minutes for flukes to detach so 3 minutes may not be enough time. If the fish is not handling the dip, then I would definitely pull them out before the 5 minutes are up.
Each fish is different, some people go longer. It is usually advised to remove the fish once any signs of distress is observed. If no stress is noticed on fish then 5 min is recommended to get as much fluke as possible or any other parasite for that matter.