Primer

The machine is created, Abe secretly builds the failsafe and starts it up, then builds his own box and tests it. Shows it to Aaron.

Aaron discovers the failsafe (because he sees the warehouse manifesto with two storage spaces under Abe's name) and uses it, bringing another box back with him to replace the original failsafe. This means Aaron is always in control, as he failsafed to the earliest point.

Then the movie begins, and we think it's the first time through, but it isn't. This Aaron is at least Aaron3 (Aaron failsafed twice). This becomes apparent through things like the earpiece, which he is using to recite events, his quick math regarding the watch in the garage, and his nod to the correct storage unit when Abe takes him there for the first time, but tries the wrong door. It cannot be Aaron2, due to Abe's linear perspective governing the movie (this is confirmed by Shane Carruth) and the fact that according to that perspective, Aaron2 left immediately after the conflict of all three Aarons in his house (drugged Aaron(1), Hoodied Aaron(2) and Sweatshirt Aaron(3).) Aaron2 is the movie's narrator, and you will notice that once he leaves, his narration about the party etc. becomes speculative.

For the rest of the movie up to the Granger incident, you are dealing with the original Abe, and Aaron 3. Abe failsafes, erasing Aaron3 back as far as Monday morning when it all began, but Aaron 3 is still intact because was already in the timeline prior to Abe's return, and is a permenant fixture in the timeline. (His original is stuck in an attic, and he is acting out the days to keep them the same.) Abe drugs his original also. Aaron3 realises here that Abe has failsafed from a point in the future and explains everything. The biggest plothole here is, none of the Aarons actually experienced the Granger event because Abe failsafed back to Monday - this is just one of the big unexplaineds. (There are theories about both dudes failsafing back AFTER the Granger event, but it's unlikely. Personally I like that theory, but it apparently doesn't work.)

Abe and Aaron complete the party mission and seperate. There are three Aarons and two Abes - one Aaron goes off and starts building what seems like a massive time machine, one disappears (the narrator) and the third is the original drugged one. One Abe is the drugged one who matches Aaron's drugged one - neither has discovered time travel yet. The other Abe remains in the area to watch them and discourage them from time travelling - though the narrator has made a phone call to someone (his narration being the call) and the general consensus is that he is informing the original drugged Aaron about time travel.

it claims that Aaron3 is indeed a post-Granger failsafe, but according to one argument (which seems to be mostly correct) he couldn't have failsafed back to the same time as Abe, as Aaron would have failsafed to the earlier point and Abe's linear perspective would be thrown out of whack (because Aaron had been failsafing multiple times, but the reason we can see Abe's linear perspective at all is because Aaron does not failsafe during the course of the first 45 minutes of the movie - at that point he has already done all his failsafing e.g he is the hero at the party etc.) We know Abe's perspective to be the 'true' timeline, as Shane Carruth has said it is. Not to mention that if Aaron three is created after the first bench scene but before the second one, there is no Aaron who could be on the bench in the original scene (the second had left town by that point and the original was drugged in the attic. If it was the original, he would not have made all the little indicators which Carruth also points out occur at the start of the movie which suggest he knows what he's doing long before Abe shows him anything.)

Personally, I want to believe Aaron 3 also failsafed back with Abe after the Granger incident, as it ties up my own support for it (the entire layout of the kitchen scene where the third Aaron is also wearing exactly the same clothing as the night of the Granger incident. The only thing I can't explain is why he doesn't have nearly four days growth of facial hair, but then neither does Abe, so maybe that was a continuity error or they shaved.) But, there is also the bench dilemma. So Aaron3 had to be the one there all along, and he did not failsafe after the Granger event.