Think in terms of a) equity over time and b) equity relative to one-another.
a) Let's say you build the release version, stick around for 1 month after launch and then leave. The others however stick around for 5 years and build a business that hires 200 people and that eventually gets bought out for 100Mio. Now what do you think is fair?
Whatever share anybody gets should vest over time.
b) Suppose you all each decide to get 200K shares (each gets a third of 600K outstanding shares issued at launch) and everybody's shares vest over time (5 years for instance). During those 5 years, you need to raise several rounds of funding and you end up selling 1.4 Mio shares to outside investors. Now the company has a total of 2Mio shares and each of you owns 10%. Everybody got diluted but overall everybody's got the same as everybody else.
Now, when you combine a) and b), I would think that everybody gets something fair and that no one would feel that they're getting shafted.
And don't go thinking that because you've built the product you've done the hardest part! Growing and running a business is actually arguably harder: there are more successful software engineers than successful software entrepreneurs!!