A third-party beneficiary, in the law of contracts, is a person who may have the right to sue on a contract, despite not having originally been an active party to the contract. This right, known as a ius quaesitum tertio, arises when the third party (tertius or alteri) is the intended beneficiary of the contract, as opposed to a mere incidental beneficiary (penitus extraneus). It vests when the third party relies on or assents to the relationship, and gives the third party the right to sue either the promisor (promittens, or performing party) or the promisee (stipulans, or anchor party) of the contract, depending on the circumstances under which the relationship was created.

Can we conclude that can a beneficiary of a trust sue a third party?
yes