The diving in Palau can not be beat. You also have a good chance to see the Mantas there too (not every day like Yap but a good chance over the week).
Yap has a lot more culture than Palau and is definately worth a stop. If you only have 3 days, you may want to book your dives ahead of time so you can dive on your arrival day and the next day. You may want to see what time they are doing the Manta dives (typically at slack tide). They also do reef dives. The 24 hour no fly thing makes 3 days pretty short but 2 half days of Mantas is still pretty good.
Yap has a lot more culture than Palau and is definately worth a stop. If you only have 3 days, you may want to book your dives ahead of time so you can dive on your arrival day and the next day. You may want to see what time they are doing the Manta dives (typically at slack tide). They also do reef dives. The 24 hour no fly thing makes 3 days pretty short but 2 half days of Mantas is still pretty good.