The preposition συν-, attached to a cardinal number, signals distributivity.
σύνδυο '(two) by two', σύντρεις 'by three'
The use of compounds with συν- is comparable to the use of the Latin distributive numerals, although the Greek construction is much less frequent.
τοὺς ἀκέων συνέεργον ἐυστρεφέεσσι λύγοισιν, | τῇς ἔπι Κύκλωψ εὗδε πέλωρ, ἀθεμίστια εἰδώς, | σύντρεις αἰνύμενος
Those [sheep] I silently tied to each other with the well-twisted withes, on which the enormous Cyclops usually slept in all his godlessness, and I took hold of them by threes.