Why does mongodb require three config servers
Config servers are a special case, you can use one (for testing), or three and that's it.  They are not a replica set, at least not yet, that might change in the future, see SERVER-1448.
Go and (re)read the first two paragraphs here - note the piece about them not being a replica set is explicitly mentioned.  I will pick out some other relevant points:

Config servers use a two-phase commit to ensure immediate consistency
and reliability
to ensure redundancy and safety in production, you should always use
three

The first part gives you a hint about why 3 are needed, the second part is telling, particularly in terms of the safety aspect (redundancy is obvious - you have 3 copies of the data).
With how the 2 phase commit is implemented, all three config servers must receive (and acknowledge) a change for it to be considered successful.  That's the safety piece - all three must be in a quorum and have the same data (there are regular checks), and that is why it is three config servers required exactly (for now).  When you run with just one for testing, this can't happen, and your cluster meta data is essentially at risk, hence using just one is for testing purposes only.