Question

Of the Romans "group of three" gods in the Archaic Triad, which one did not have a Greek counterpart?

Answer 1

The Archaic Triad consisted of Jupiter, Mars, and Quirinus. The Greek counterpart of Jupiter was Zeus, and the Greek counterpart of Mars was Ares.
Sources: http://en.wikipedia.org/wiki/Capitoline_Triad#Archaic_Triad and my own knowledge of Greek Mythology