Sammy 1 =
First Sammy is just a guy from Lennys past to remind himself he's got CRS. Sammy was a fraud (not maried) pretending to have CRS. Lenny exposed him and got a big promotion (all said by Lenny in the movie).
Sammy 2 =
During and after Lennys first mental hospital (after his second high school love Catherine, a girl he wanted to marry but refused so he trew her out of the car, got 'hit from behind' by another car (the last thing Lenny remembers) and got CRS, overdosed on drugs she stole from her last victim, a drug dealer killed by Lenny because his 'love' (now gone bad and abusing Lenny) said he was raping her) Lenny uses Sammy as a 'coat rack'. He now believes Sammy wasn't pretending, but really had CRS (and a wife). His own wife OD-ing is replaced by Sammy OD-ing his wife unintentionally because Sammy was unorganized so he ended up in a mental hospital. It reminds Lenny to be organized (carrying a dossier, notes, photo's, tattoos) and that he should fool the doctors in the second mental hospital ("You bluff it to get a pat on the head from the doctors. You bluff it to seem less of a freak").
(He also fools Natalie, his first high school love who tries to 'heal' him and get him out of the second mental hospital ("So good old Leonard Shelby from the insurance company gives her the seed of doubt, just like he gave it to the doctors"). She succeeds and he gets released, but when they have sex for the first time in her house he accidently strangles her in the bathroom with the shower curtain, thinking she is the rapist/murderer he is looking for. So in his third mental hospital they now keep him in a straight jacket ("If I could just reach out and touch her side of the bed I could know that it was cold, but I can't.))
Sammy 3 =
During Lennys third (and final) mental hospital he has this short moment of insight in which he sees the truth (he is a random killer being abused by many and Sammys story is a lie. Sammy was a fraud.) (this is a 'hidden' scene, not displayed in the movie, but described in Jonathans short story Memeto Mori: "Moments of clarity, insight, whatever you want to call them. The clouds part, the planets get in a neat little line, and everything becomes obvious." - In my FRAME in my post "The true chronological story of Memento" this is the middle of scene 18 = 6+6+6 = the number of the beast. Instead of accepting the truth Lenny decides the only time in live he was happy was when he was chasing the alledged the rapist/killer of his wife ("I know I can't have her back, but I want to be able to let her go. I don't want to wake up every morning thinking she's still here then realizing that she's not"). So he makes a note for himself (the mirrored tattoo), kills the gay warden (a guy called John G) who is raping him in the third mental hospital (happened to Uma Thurman in Kill Bill too) and sets out on an endless quest for murder.
Sammy is now a secret. Lenny doesn't talk about him anymore and he doesn't want people to know he's got CRS. He no longer uses notes but is 'acting on instinct', reviving his hunger for revenge by using hookers to warm the bed beside him and start looking for a new rapist/urderer the next day.
As I said: the whole story is depicted in my FRAME in aformentioned post. I have also posted many replies to many other post, explaining more about the details (as the frame doesn't mention the specifics of all murders). The most important clue is the opening shot which hands you to five 'friends' Lenny has during the story. If you search for all my earlier replies and lay them beside my FRAME you should get the story. Try it and let me know, because NOBODY SEEMS TO GET IT !!!!!!!!