In order to eliminate three blocks, they have to be all in a row, either vertically or horizontally. Any block of the same color that is connected to the matching row will be eliminated, as well. Try to stack as many of the same colored block together as you can before using up the fourth block in the current batch. To get rid of the white X blocks make a bomb combo near them, and they will completely disappear. On later levels you'll have falling white-X blocks and there is nothing that you can do with them but to work around them. The power ups appear on the screen as you play every now and then and they will scroll by but if you don't get them, they will be gone forever. Don't tap on the ones in red, as these are booby traps. If you accidentally screw up and drop a block before you change it to the color configuration that you want to, you can tap the screen as it's falling and it will switch around again.