Reference for Bekhorot 77:17

ובלבד שיהו משולשים

R'Ashi inquired: If you will say that [it is preferable] in a field, what is the ruling as regards a garden adjace to a field?<span class="x" onmousemove="('comment',' Where the animal is fed with fodder (fresh and dry) for a cure. Does it enjoy the air here as well as in a field?');">¹⁰</span>

Jastrow