Reference for Zevachim 148:1

מ"ש שלשה דאיכא רובא שנים נמי איכא רובא מאי ג' דקתני תרתי והוא

<br> Why are three different? [presumably] because there is a majority? Then [if it fell] among two, there is also a majority? - What does he mean by 'three'? two together with itself. Alternatively, he agrees with R. Eliezer. <br>

Jastrow