A beneficiary can sell their interest, so you could sell your 1/3 share. However, you would continue to be 1 of 3 trustees, with your sisters, but would now be holding the property on trust for your 2 sisters and the new buyer. The buyer would not be able to get a mortgage for this purchase, and it is unlikely that a third party would want to get themselves involved for the same reasons that you don't want to. Even if you could find a buyer and you got your money you would still be involved as a legal owner / trustee, and would have a legal duty to look after the trust property in the best interests of all the beneficiaries.

It is irrelevant whether the beneficial interest is held as joint tenants or tenants in common. If a joint tenant attempts to sell their share the joint tenancy is automatically severed and converted to a tenancy in common.

In practise you and the sister in agreement are better of applying to the court for an order for sale. This is available in any trust of land where the trustees cannot agree. The court are likely to order a sale unless there is an overriding reason not to, and in the circumstances there does not appear to be any such reasons. If you are thinking about this you need to talk to a solicitor, but hopefully the real threat of this will persuade the other sister to agree to a sale ASAP.