All three sentences are grammatically incorrect as the verb "stattfinden" isn't transitive.
However that kind of misuse of participles is common and accepted in legal and bureaucratic style.
In the first sentence "stattgefunden" appears strictly redundant. It is exactly equivalent to the grammatically correct:
Der Ausschuss beschloss in seiner Sitzung vom/am 12.10.2014 ...
In the second and third sentences "stattgefunden" is a separate attribute to "Sitzung" which stresses the fact that the meeting in question has indeed taken place, probably in contrast to another which was cancelled.
The difference between "vom" and "am" is marginal.