Luffy's tattoo is 3D2Y which he made it for a reason. The 3D crossed out with an X means not 3 days and 2 d means 2 days. He did the tattoo to communicate with his crew members because luffy "the captain" had made some change of plans for the safety of his crew.The reason that Luffy made a tattoo on his right arm and 3D2Y because his crew members where all in different places in the world, not only in yhe first half of the red line. Before they all came apart, they had agreed that they will meet again in 3 days at Sabaody in a specific bar. The captain "luffy" made some change of plans because if they meet in three days the marines and the admirals will attack them, so Luffy decided that all of them will meet in 2 years and not in 3 days. So he made a tattoo that the marines will not notice and only his crew will understand it.