My 3RD Unknown Ancient Coin

It's a sadly beat-up example of an early Imperial bronze, from emperor Tiberius. The reverse legend that is puzzling you begins at the top, and would read PONTIF MAXIM TRIBVN POTEST XVII if the coin were fully intact. It's a good thing the "XVII" is readable; this is actually a date, the 17th year of tribunician power of Tiberius, equating to late 22 or early 23 AD. Example on wildwinds, example on CoinsCatalog.com.

"SC" stands for "Senatus consulto" - Latin for "by order of the Senate". Under the early Imperial monetary system, gold and silver coins throughout the Empire were issued under the personal authority of the emperor, whereas bronze coins were issued under local city authorities. In Rome itself, that authority was the Roman Senate, though you can also find "SC" on some "provincial bronzes" issued by the city authorities in other large cities (such as Antioch).

As for coins of Antoninus Pius, he was emperor a hundred years after Tiberius; the artwork on the coins, and most especially the style of the lettering, changed dramatically in the intervening century. Compare the lettering on your coin (and on the ones in the above links) with the lettering on a bronze coin of Antoninus Pius, such as this one. The lettering on the later coin is much smaller, and the serifs are relatively much larger.

Technically, SPQR is abbreviation for "senatus populusque romanus", meaning "the Senate and the people of Rome". And yes, it is found on numerous Roman coin types, dating from the Republic period to some Late Roman bronzes of the time of Constantine. Simply go to Wildwinds partial inscription search engine, type in "SPQR" and you'll see lots of them. Most common are the numerous types, in all denomninations, of SPQR OPTIMO PRINCIPI coins of emperor Trajan, such as this one.

It remained a distinctive symbol of the city of Rome, long after the fall of the Empire. Mediaeval coins from the city of Rome, issued under the authority of the hereditary Senator rather than the Pope, bear these letters on the coat of arms. And it's on the coat of arms of the modern city authority.