hereiam

Note that there really isn't a "three-year thing". That was a more or less arbitrary line drawn by the MTC branch president. He may have received instruction to that effect from the mission president, but there is nothing in the Church handbooks that specifies three years as a magical number past which sins don't count as much. That is not the way God's law works; there is no "statute of limitations" on the damage done to one's spirit by sin.
The point of repentance is to repair spiritual damage and get yourself on the track you need to be on. Don't worry or even think about three-year anything. Just talk to your bishop and let him guide you.

@hereiam,
While @pam is correct that your Bishop is the steward over you, in my personal experience, if it has been more than 3 years since the offense was committed, I would not expect a great deal of disciplinary action to be involved.  When I was in the MTC, the Branch President asked me in an interview if I had engaged in sexual intercourse, of any kind, at any point within the last three years.  He informed me that if it was longer than three years prior, I would still need to confess, but would not be sent home.  It was an easy question for me because I had never done that, but I found the three year stipulation to be informative.  It was clear from our discussion that he did not create that time period arbitrarily, but was directed by higher authorities to use the three year rule.
If there is some merit to this rule beyond the MTC, I would presume that being racked with the knowledge of your sin during that time may be considered discipline enough.  If you confess to the Bishop, perhaps you will be required to abstain from the sacrament, perhaps more, it all depends on your individual circumstances and whether or not you are and have been truly penitent.

Thank you all for the kind words and honest truths, much appreciated.

Thank you all for the kind words and honest truths, much appreciated.

http://www.mormonnewsroom.org/article/church-discipline
https://www.lds.org/ensign/1990/09/a-chance-to-start-over-church-disciplinary-councils-and-the-restoration-of-blessings?lang=eng

Hi @hereiam it's good to hear from you. As others have said run, don't walk to your bishop's office. One of the greatest lies Satan tells us is that repentance is harder than hiding your sin. Don't listen to that nonsense! Depending on the severity of your sin, you may have to face some form of church discipline but the instant you talk to your bishop, you will feel an immense level of relief. Nothing you do while repenting will equal the pain and fear you've felt trying to hide your sin. The very moment you speak to your Bishop, the Savior's Atoning power will begin working for you, to cleanse you of your sin. No matter what you have to go through, Jesus Christ will be with you every step of the way, and you will find the peace the Lord promises in Isaiah 1:18 "Come now, and let us reason together, saith the Lord: though your sins be as scarlet, they shall be as white as snow; though they be red like crimson, they shall be as wool". 

Go see your bishop - the result of that will be better than not going (regardless of either result, 100% guaranteed).  It'll be OK, just go see the bishop.

The only one that can answer this is your Bishop.  He is the one with stewardship over you.  Not a bunch of random people on the internet.