I mean, what did 8th-9th century Muslims say instead of "Islam," "Christianity," and "Judaism"? For Christianity and Judaism, was it something like "Nasiriyya" or "Yahudiyya"? (those are just guesses)
What did 8th-9th century Muslims use as the word for their own religion? Was it "islam"?
Do you mean specifically those religions, i.e. when one is talking specifically about Christianity distinct from Judaism and Islam; and Judaism distinct from Christianity and Islam; or the Abrahamic religions together?
For the term "Islam", it's always been known as Islam, as evidenced in the Qur'an. In Suah 3, verse 19 the term الإسلام (al-islam) is used. Furthermore in Surah 3, verse 67 you have the terms for "Christian" (نَصۡرَانِيًّ۬ا - nasraniyyan) and "Jew" (يَہُودِيًّ۬ا - yahoodiyyan).
For the Abrahamic religions together, there is a special term, Ahl al-Kitab ( أهل الكتاب) which means "People of the Book" - it narrowly refers to Christians, Jews, and Sabians together, more broadly including Zoroastrians as well. According to Surah 22, verse 17 they will all be distinguished [from polytheists] on the day of judgement.