Profit!

From CGWiki

The 3-step path to untold riches goes like this:

1. <Insert any activity you want>

2. ???

3. Profit!

In Comic Genesis, the first step is usually "Draw an online comic". This highlights the fact that a lot of people start up webcomics expecting to get rich, but basically no one ever does.

The second step from the end is not ambiguous. While other steps may be between the first and the second-to-last, the joke doesn't work if the second-to-last item isn't "???".

Earliers uses of this phrase can be thacked down to here and here, it can be assumed that those were the origins of the phrase on Comic Genesis.

The phrase itself originates from a South Park episode which featured the "underpants gnomes". They stole underpants from people as part of a similar scheme to get rich. In the original joke, "Steal Underpants" was step 1 of the 3-step system.

Probably the oldest catchphrase still actively used.