Epic 40k (E40k) is often used as a catch all term to refer to all GW 6mm scifi epic. However, in rules terms it specifically refers to the 1997 3rd edition of epic. This system diverged a lot from the original Space Marine rules and fell out of favour. It's got some innovative and abstract rules mechanics but is not widely played (though some fans of it on these forums!).
NetEpic is a community run ruleset that follows on from the Space Marine era of rules from the early 90s.
Epic Armageddon (EA) is the most recent version of Epic. Less abstract than Epic 40k but faster play (though with smaller armies) than SM/NetEpic). Though now out of production along with the above 2 rules, it has the largest continuing player base and tournaments around the world run by the community. The lists and rules have been updated by the community under the Net Epic Armageddon project here
https://www.net-armageddon.org/(If you are French, there's also the french rules/lists following on the Epic armageddon, for use in french tournaments, the lists are quite different. In the UK Epic UK runs tournaments using Epic Armageddon, some of the lists are a bit more locked down than Net Epic Armageddon for tournament balance).
So a bit of a mix of names and bits of names being used to apply to a mix of things! EA is probably the most played, especially in terms of organised tournament play. NetEpic also has an active community and some events. E40k is probably the least played, but the rules are worth a read!