The requirement to move to the US with an E3 is to hold the equivalent of a 4 Year Bachelor Degree (or greater). In some cases you may be told to seek equivalency from an evaluation company in order to prove this (or at the very least, use it as a precaution). As a general rule of thumb, three years work equates to one year of education (who knows how that rule was come to).
I did mine through Silvergate Evaluations who turned out to be great. You send them everything you have, and they will reply with options on who can write your equivalency statement from a University or College in the US. Prices will vary but at the very least you'll be looking around $250USD for the full document.