b2evolution.net uses all 3 but they're not integrated. We just replicated the design. All 3 applications are actually running on different servers in our case in order to balance server load.

The wiki only uses username and password from the phpBB install. That's fairly easty. If you would incorporate B2evo you could do something similar.
However, think of these points.
- What does it gain? Looks like only a few clicks for the member
- You will never be able to link the data. You can't show in phpBB how many blog posts I made. If you ban some one from the forum he will still be able to post in the Wiki. Etc etc.

It's concederations like these that make things akward and hence the reason it isn't been done before.

As fplanque described you can make a header that is the same for all three applications. You will end up with an install like this one.

Good luck