EPC allows ordination of women to all offices on a local-option basis and permits speaking in tongues. A lot of local autonomy on 'non-essentials' that some folks here would classify as essentials.
OPC is the smallest and most homogeneous of the 3 mentioned. Has a reputation, historically not entirely undeserved, of being less aggressive in expansion, more inwardly focused on purity. They are the most orthodox of the 3 mentioned.
PCA is the largest and the most mixed bag, with emphasis on mission over purity, to a large extent. (Although some Presbyteries strive to maintain the standards.)
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Origins: Founders of the OPC broke from the northern church (PCUSA pre UPUSA), PCA broke from the Southern church (PCUS), and the EPC broke from re-united PCUSA.