There is some confusion over the identity of the largest island in a lake on an island in a lake on an island.
Elbruz.org has long had a nice piece on recursive islands and lakes, but they get the final one wrong, saying that the island in Taal’s Volcano Island’s crater lake is the largest island in a lake on an island in a lake on an island.
That Filipino “triple island” is only about 1 acre / 0.4 hectare.
The largest known island of this sort (subject to further exploration; some Arctic areas are poorly mapped) is the largest island in the largest lake on Glover Island, in Grand Lake on Newfoundland.
Glover Island, the world’s second-largest island on an island, has a many lakes on it, and the largest has about 17 islands. The largest of these is 2 acres / 0.8 ha. When I visited it, it supported its own little patch of woods.