3 .. no big deal. I have been there when there were 6 ships in port. It was after a hurricane had gone through and all the ships had switched up their itineraries to avoid the storm. Grand Cayman had all their tenders running and it still wasn't enough. Luckily the Princess ship in port was able to use its own life boats as tenders so that provided some relief.
As far as the signage goes, yes they do a great job at helping you find the correct line to stand in. You should not have a problem.
Carnival Cruise Lines:
Fantasy 90, Celebration 95, Victory 05, Conquest 05, Spirit 6/06, Freedom 6/08, Holiday 7/08, Victory 6/09, Valor 9/09, Dream 6/10, Splendor 8/10,Conquest 8/11,Magic 11/11,Triumph 4/12,Spirit 7/12,Freedom 11/12, Triumph 2/13 - The fire cruise
NCL Dream October 2006, RCCL Mariner OTS June 2007