Difference of 不可. 不要. and 别
不可, in the context of your question, is short for 不可以, which means "can't", not "don't".

不可吃 means "can't eat", perhaps it is inedible or passed expiry date.

不要 and 别 both mean "don't", but 不要 is stronger and more instructive than 别, example:


  公共场所，请不要喧哗 (please don't make loud noises in public spaces)


不要 can be placed at the end of a sentence to mean "don't want". It can also be placed before a noun. 别 requires a verb behind it. You can say:


  这东西我不要 (I don't want this thing)
  我不要这东西 (same as above)