The F6F-5 was introduced about the same time that the US Navy switched from the tri-tone scheme to overall glossy sea blue per Bert Kinzey's F6F Hellcat in Detail and Scale. It is certainly possible that squadrons currently outfitted with F6F-3s may have painted their birds overall glossy sea blue but I would lean toward that not being the case.
For instance, in the book I mention (pg. 45, upper left), there is a photo of a very early -5 that still had the small window behind the cockpit that was a feature of the -3 with paint that appears to be overall glossy sea blue (hard to say for certain given it is a black-and-white photo, but I see no demarcation lines anywhere in the picture).