Certainly there are ways! It gets a little complicated, though, when you are contributing to so many projects.

First, the reason why only one project runs at once: these science applications are not light weight! The requirements just for a single project are so high that running three at once would make your computer unusable. And that's not helpful, so BOINC runs one at once.

Here's a quick summary:
Predictor@home is doing structure prediction.
Rosetta@home is developing better structure prediction.
WCG has two projects:
Human Proteome Folding is doing structure prediction using Rosetta.
FightAIDS@Home is searching for candidate drug molecules against HIV Protease.

Now, chosing which BOINC projects you want to work on is as simple as attaching or detaching the project. Do it from BOINC Manager, on the "Project" tab. Deciding what weight to give each project is more complicated. You have to go to each project website, and change your BOINC preferences.

If you want to know more about what each project does, then read about them on each project's website. It can get into some pretty heavy science very quickly, though. Start by clicking on the Research button up at the top, to learn more about the World Community Grid projects.