I commited to 3 different Area51 proposals. Now I saw that Sports is in the commitment phase and I want to commit there too. But the limit is set to 3. So how can I uncommit from a Area51 proposal?
You can undo a commitment at any time as long as the site doesn't exist, in other words as long as the proposal is still in the commitment phase. Once a beta site is created, it is time to follow through on your commitment.
Looking at your profile, you are committed to three proposals that are in the beta stage. If you want to commit to other sites, you must first fulfill your commitment to at least one of the existing sites. You would do that by posting questions and answers on the sites.
If you don't choose to participate, you're proving that you weren't serious about your commitments. So it's understandable that you shouldn't be allowed to commit to more sites — if you didn't follow through in the past, you're unlikely to follow through in the future. Nonetheless, you will be given a reprieve. Once a site leaves the beta phase, you will get back the associated commitment slot. Until then, your help is still needed to build the site.
First, uncommit. Just go to the proposal you want uncommit from and press "Uncommit" (this only works for proposals in the commitment phase, not for proposals in a beta phase):
Commit to the other proposal. Press "Commit".
In the first part of Area 51's life it was not possible to uncommit from a proposal until the public beta was over (this could take a loooong time if the proposal moved at glacial speed through the different phases), but it is possible now.
If all three committed-to proposals are in a beta phase you either have to wait until the beta phase ends for one of them or follow through on the commitment (by posing questions, answering questions, commenting, etc.) for a proposal.