The phrase "a third subject" is correct and usable in written English.
It can be used when referring to an additional topic or participant in a discussion or analysis, particularly when two subjects have already been mentioned.
Example: "In our study, we focused on two main subjects, but we also considered a third subject to provide a more comprehensive view."
Alternatives: "an additional subject" or "a tertiary subject".
Next to Mycenaean and Anatolian philology, a third subject now caught her attention: the history of her own discipline.
Politics and religion are the two topics that you are traditionally supposed to steer clear of in polite conversation, but there is a third subject that the British also rarely discuss: how much we are paid for our jobs.
She was educated at James Allen's girls' school in Dulwich, south-east London, before going on to King's College London, which she disliked as much as she disliked her general degree course in French, history, and a third subject which she was so unenthusiastic about that she later professed to have forgotten what it was.
A team led by Quinlan and Ben Backus, of the SUNY College of Optometry, had secured National Institutes of Health funding for a human trial, but, after a search lasting months, Amy and Carl were the only qualified volunteers, aside from a third subject who dropped out after thinking twice about the treatment: living with total strangers, in complete darkness, for ten days.
A group is a mathematical structure consisting of a set of objects together with a binary operation, i.e., an operation combining any two objects to a third, subject to certain requirements.
A third subject (Arn) initially gave all four raisins, but after the 16th set of two sessions, he learned to give 1 2 raisins to both partners.
Any disagreement was resolved by consulting a third subject-matter specialist.
"A second subject fled from interrogation in panic".
A second subject, the "observer," sat across from the first subject and watched the experiment unfold.
Publishing courses – some of which are offered in conjunction with a second subject – will teach you about the editorial processes, design, marketing and business surrounding anything from magazines to digital texts.
