"Voglio pane però io non voglio acqua."
The difference is that ma=but/however/yet, pero=pear tree, and però=but/however/yet... :)
Ma and però are generally interchangeable. The simple answer is that "ma" is found at the beginning of a phrase, such as I do not want water/non voglio acqua.
The more complex answer is that però can be an exclamation, can be placed at the end or the middle of a phrase, and can have a more emphatic use.
You could say "Voglio pane. Non voglio acqua, però." meaning "I want bread. I do not want water, however."
Or even "Non voglio acqua." and somebody responds "Però!" meaning "Would you look at that! He doesn't want water..."