I think I can help with the whole AC B and AC III thing.
It will go:
Assassin's Creed I
Assassin's Creed II
Assassin's Creed Brotherhood
Assassin's Creed III
There is no such thing as an Assassin's Creed IV, or at least it hasn't been confirmed yet. The main series ('main' meaning it follows Desmond) was originally meant to be a Trilogy. However, Brotherhood was 'squeezed' in there.
Also note the fact that it is in fact the third official installment of the main series, however it is named Assassin's Creed Brotherhood. Assassin's Creed III is the fourth installment, however it is named Assassin's Creed III.
Also take into account what Ubisoft said once. Any Assassin's Creed game with a number (I, II, III) has a new Assassin and possibly a new setting. Any Assassin's Creed game with a word or phrase (Brotherhood) features an Assassin who has already been used.
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Now I know this is off-topic, but does anyone here know of an UNOFFICIAL Assassin's Creed Forum? With a good community?