Source: TW
Revised: Numbers in the RV. Note that they are mostly “Benfordian” except for 3 and 7. These were clearly over-represented by ~3-5 times suggesting that they had a special significance. A whole sUkta to the ashvin-s 1.34 of hiraNyastUpa is one of triads.
3
3 comes in several key compounds such as tridhAtu: It means 3 divisions or parts. This term was perhaps ancestral to the later trihumoral theory of traditional medicine given a mantra to the ashvin-s:
" trir no ashvinA divyAni bheShajA …tridhAtu sharma vahataM shubhas patI"
Another common compound trivandhura (10x) is also strongly associated with the car of the ashvin-s.
The other common compound is trivarutha (11x) meaning triple fortification. This brings to mind the circular & square fortifications associated with the Sintastha-Andronovo complex.
Another is tripR^iShTha: with 3 surfaces or 3 backs – this is usually applied to the god soma. I’m still unclear about the metaphor but it probably implies the pervasion of the 3 realms. 3 also appears in the name of the god trita with deeper IE roots, e.g., Greek Triton
7
As for 7 we have some similar compounds as 3: tri-chakra : sapta-chakra. Whereas the former is the car of the ashvin-s, the later is associated with the car of somApuShaNA. We also have tri-dhAtu & sapta-dhAtu as parallels. However, saptAsya or saptashiras are divine – agni & bR^ihaspati.
While agni is sometimes seen as 3-headed, the most common tricephalic entity is the demonic trishiras tvAShTra. Thus, the divine hotR^i bR^ihaspati is 7-headed while the partisan of the dAnava-s is 3 headed. Similarly we have 7 hotR^i-s or R^iShi-s; 7 mothers etc.
Some of the tripartation of the demonic is shared with the Greek exemplars like Geryon and Kerberos. The 7 heads of agni was probably associated with 7 Pleiades – a number preferred in archaic records.+++(5)+++
The 7 heads of bR^ihaspati parallels the 7-headed chant mentioned more than once in the RV. Sometimes it is called the 7-headed 3-fold chant. This metaphor is not entirely clear to me but perhaps refers to the sAman with its 7 tones?