Level 3

This one is way too complicated to explain here. It would take up a huge amount of space, and is probably already on this site several times in the "Walkthroughs" section. Seeing as there are four walkthroughs and a checklist on the page, just go over there and read it from one of those.

Http://www.supercheats.com/gamecube/supermariosunshinewalkthroughs.htm

If this is the one with all the fire goop, then you go to the right side of the island, then swim through the little river all the way to near the huge tree. Get sopping wet, jump out- close to the goop, but not in it- and the water dripping off you will wipe away the goop. Sometimes more may drop down. Just keep doing it. Go as far as you possibly can with the water, then spin jump into the goop-less area. Get flood, clen most of the goop, stop the burning mushrooms, then get onto the golden mushroom in whatever way you see fit. Clean off mayor, and PRESTO!
Shine sprite.

Cross the bridge, ignoring the ghosts that come after you.
Go around the perimeter, to the right, until you're at the back of the village. Get below the flip gate near the brown Pianta.
Clinging to the underside of the walkway, avoid 3-4 enemies and climb onto the brown gate. Hit it with B and at the other side press A and move away before it falls down. Avoid 3 enemies here and drop (B) onto swing. Jump to large mushroom and go across ropes heading south. At bouncing, green mushroom, you can either go through the flip gate and get a 1UP mushroom above, or just bounce with A to cling onto walkway above. Climb onto brown gate, hit it with B and get off it with A. Avoid 3 enemies here and go up flip gate with A.
Jump wall-to-wall up square hole. Then jump to crate, then to coins, until you arrive at base of wooden fort. Jump up and get FLUDD back. Go to the top, face mushroom, then backflip and hover backwards to get atop yellow mushroom to clean Mayor.