Towards the end of Yoshi's Island 3, after a few of those rotating-sinking wooden platforms, there's a green block with a white star on it. Hitting it gives a coin (and it briefly turns into a question block just as the coin comes out). I think there are some other blocks like this, but they just seem to act like regular coin blocks. So why do they look different?

the green block with the white star is a bonus at the end of certain stages, the number of coins collected in a stage will change the coin to a free mario

that never worked for me

This particular block will sprout a 1-Up Mushroom if you collect 30 or more coins in a particular level. I believe that enemies that are counted as coins through the "other" methods of getting coins would count towards this total; fairly confident but not 100% sure. By "other methods", I'm referring to Fire Mario, after Yoshi gets a Red Shell in his mouth, a Red Yoshi gets any shell in his mouth (spitting it out in both instances in which a shell is involved), or Yoshi eating a berry/normal enemy (such as a Piranha Plant, Monty Mole, Goomba, or a Koopa without his shell, for examples).
But the point is, once you get 30 or more coins in that particular level, a 1-Up Mushroom will sprout from this type of block.

I thought it was just 30 coins,not from the level.But it doesn't matter since there is easier means to get 1-ups.

It's true, 30 coins in that particular level only will give you a 1-up. Any thing less will give you an extra coin. Just because your coin counter reads 30 or more doesn't mean you'll get the life, nor does it mean you won't get the life because it just so happens to read under 30. To revisit elementary school, entering a level with 40 coins means you need at least 70, entering with 90 means you'll need at least 20, so on and so forth.

if you have 30+ coins youll get a 1-up mushroom.