Ultimately, a win in the green card lottery is just a ticket to "apply" for the green card itself. In no way is it a done deal that you will receive one. Hence why they always draw more than the allocated allotment of green cards ... because a lot of applications will be rejected.
Very true. The fun, stress and financial burdens really get started after you get selected.
Anyone can "apply" for the lottery however you still need to meet prerequisites after "winning". From what I've heard/read, a long, drawn out process once you do win, and this was a few years before the Dotard was in charge. Who knows what its like now.
Prerequisites are finishing highschool and having enough money to support yourself.
It's a pretty big risk to expect to transfer across to a green card from an E3, although it can has been done.
The lottery is won independent of any other visa. You can enter it in Australia, never having been to the USA before. Should one marry a US citizen on an E3, then get a green card, it's the same kind of thing. You are becoming eligible because of outside forces not related to your current visa. It's not really a risk to apply "from" and E3, you are not really converting anything, you are changing your status because your eligibility has changed, just like in the marriage situation.
The real risks are, if you win and begin the process, then they run out of visas or you for some reason get denied a green card, you will revert back to your E3 status. Having shown immigrant intent, you are unlikely to be given a renewal, as the E3 is nonimmigrant.
Once you apply to adjust status you can't leave the country, so no Canada or Mexico flying. You need to wait for the issuance of the green card before you can travel again or else they will consider your application abandoned. If you need to travel, you can apply for a document that lets you do it, but that will essentially cancel your E3. If you are unsuccessful in the green card, then you'll have to go home.
I decided that the risks were worth it because the USA is an amazing opportunity, winning the lottery is huge and I'm in a position where they will not run out of visas before I am processed.