If i want to highlight my hair in 3 colors,should i first bleach all of my hair?i have mahagoni color now.?

Not sure what colour you're looking to highlight, but if you want to keep the mahogany as your base colour, I wouldn't bother bleaching your hair, it's really really bad for it! Get someone to help and use foil or a highlighting cap to keep each section of hair away from each other while you highlight. A tinting brush may also help.