How do i fix a 3year relationship filled with lies, betrayal, infidelity ?

I don't think it's possible anymore. A pair of errors can be passed on, but as you said there were lots of problems!! As you can love your bf you can love many others who maybe won't make the relationship with you a disaster!

Its easy to say but hard to do. If you still love him/her then start a "commitment" to each other. Marriage is not a answer but the commitment and respect for each other. If you love and respect him/her, it might work out and end for a life time but if you just love him/her and you don't respect him/her, there is a problem between you. Fix it before time flies and you can never turn back.

U no i heard this while watching TVSome old lady said this things that starts with lies have to end with lies and in between if truth comes in between it makes it go in a zig zag way.SO if u tell the truth be preapre to suffer for a time.

you don't have to fix it. you must go out of that relationship. i guesss this is the right time to end it while you still love each other and there's still respect for one another. You may end up as good friends. Rather than wait for the time when your heart is filled with hate and bitterness because of the lies, betrayal.