Magic Key is the 3rd level. and the first one that is a level difficult. not too hard though.
how: jump to the far left, and then just like the second lvl glide to the right to the entrance. do not touch the milk. jump onto one of the four pads going in a circle. but you can not get to the black diamond teleporter! these blocks with key wholes on it stops you! so then, jump down to get the key, and don't touch the spikes *cough* obviously *cough*. then the blocks go away, and then continue to the black diamond don't touch the milk just before the diamond.
crown: once you are on the four rotating pads, wait for your pad to go all the way up, and you will get the crown. or be impatient and jump your way there to speed the process.