The first oath was to the Commonwealth, the second to the US Constitution and this the third and final, hence the celebration at the end
Section 38. Whoever is admitted as an attorney shall in open court take and subscribe the oaths to support the constitution of the United States and of the commonwealth; and the following oath of office shall be administered to and subscribed by him:
I (repeat the name) solemnly swear that I will do no falsehood, nor consent to the doing of any in court; I will not wittingly or willingly promote or sue any false, groundless or unlawful suit, nor give aid or consent to the same; I will delay no man for lucre or malice; but I will conduct myself in the office of an attorney within the courts according to the best of my knowledge and discretion, and with all good fidelity as well to the courts as my clients. So help me God.
Source of oath: