Validity: From premise 3, a necessary being's existence comes from itself or another; from premise 2, if from another, an infinite regress results (which is unsatisfactory and thus rejected); therefore, by disjunctive syllogism, there must be a necessary being whose existence comes from itself—i.e., whose necessity is uncaused.
Confidence: Clearly reconstructed from the text's summary of Aquinas's Third Way argument.