You can sell them your interest for one dollar if you want to (makes no sense to me to do that), but you cannot remove yourself from the responsibility you took on with that mortgage.
There is only one way to get yourself out of the mortgage, and that is for the other two to refinance the existing mortgage in their names. You should be asking them to refinance the existing mortgage AND pay you your share in any value above the existing mortgage balance. Maybe they won't want to, maybe they can't afford to. But you are legally entitled to your share.
If you wanted to sell and the other two did not, and they could not afford to refinance without you or the three of you could not reach an agreement on what's a fair splitting of the equity, you COULD file a partition lawsuit, and a judge would ORDER that the property be sold and the net proceeds split.
I know that's not what you want to do; I'm just telling you that's what the law allows you to do.
So, if they are able to refinance without you, that will get you out of the mortgage debt. It's something all of you should speak about with a lawyer with tax experience. If you give away for $1 some value that is more, it can have future tax implications for them when they sell, and you may have to file a gift tax return, if the value you are giving up is substantial.