Nearly all commentators on Brahma Sutra (including Shankara and Ramanuja) defended animal sacrifice in the limited context of a Vedic Yajna
Though it is v v likely Shankara / Ramanuja were vegetarians
And that ethical argument did not always merely stem from the two nAstika maThas, but often from within the Astika fold
"If one offers to Me with devotion a leaf, a flower, a fruit, or even water, I delightfully partake of that article offered with love by My devotee in pure consciousness"
This eventually led to the emphasis on pUja in pANcharAtra religion, and the widespread popularity of "vegetarian" offerings
Having said that even the greatest votaries of pAncharAtra, (e.g. Ramanuja) defended animal sacrifice, suggesting that the sacrifice does not constitute violence but liberation to the offered animal (commentary on the B.S. 3.1.25)
Some commentators (e.g. mAdhva) recommended Pista Pashu offerings (animals made of flour)
That might not please the purists
So there is an injunction - "vasantAya kapinjalAnAlabheta"
"Let the sacrificer bring kapinjala birds to the sacrifice"
It can range anywhere from 3 to quadrillion
2 is not considered given that in sanskrit "plural" denotes 3 and above
One can legitimately cite the "general" Vedic injunction against injuring animals -
"mAhimsyat Sarva bhutAni" (never injure a living thing)
We combine the two, to have the best of both worlds, by limiting the sacrifice to exactly 3 birds
3 and no more
A classic example of how animal sacrifice is moderated without deviating from the Vedic injunctions