Some changes happened since the original answer by Robert Cartaino came, so it's little different now:
Is committing to your own proposal assumed, or does it use up one of
the committments you're allowed?
This is the same as Robert Cartaino wrote.
Is there ever a time you are "refunded" a commitment that you can then
use to commit to another proposal? Say, for example, you commit to a
site and it goes live (or even to beta). Things are going great. Can
you then go back and commit to another proposal to help it get
You have to fulfill your proposal while it's in beta, usually through asking 10 posts there, but there are alternatives. See details here.
Is the commitment allowance hard set at 3?
There is sort of an exception to this rule when the proposals gets closed during the commitment phase. It's not fulfilled, but it doesn't count to the three commitment tokens, so you can have four or more commitments temporarily until it is deleted or or is reopened and leaves beta. See details here.