"What ever" - is used to show a slow motion, about you not caring, but you have that tiny care in you.
"Whatever" - to imply that you do NOT care, in a rude and offensive manner.
"What-ever" - usually would be used as a special connection, to implay an idea or the common meaning. Usually used in literature.
All of them are correct, btw.
So, be more sincere, buddy.