Take it back to where you purchased it, or to a corporate sprint store. They will help you figure it out. Since you are still within your first 30 days, they might just exchange it for a new one.

 

i'm guessing that being only 3 days old you're not rooted??? what is your current prl??? before going back in, one thing i could suggest just off the top of my head, try a battery pull... leave it out 15 min... then see what happens... did you activate the phone yourself or was it at a store??? try a factory reset...
menu / settings / sd & phone storage / factory data reset...
or try this...
http://androidforums.com/evo-4g-tips-tricks/280082-major-solution-fix-your-slow-3g-speeds-evo4g.html

these things you might have tried already... just trying things i might know to help before you have to go back into a store...