Reference for Menachot 160:6
נתכפר בשלישית שניה תמות וראשונה תרעה נתכפר באמצעית שתיהן ימותו אביי אמר אפילו נתכפר באחת מהן שתיהן ימותו כולהו חליפין דהדדי נינהו
R'Zera said, And so it is, too, with regard to the sin-offering. Thus if a man set apart [an animal for] hi sin-offering and it was lost, and he set apart a second animal in its stead and that too was lost, and then he set apart a third in its stead, and then the first [animals] were found so that now all three animals stand before us, - if he obtained atonement by the first animal, the second must be left to die<span class="x" onmousemove="('comment',' For the owner of this sin-offering has obtained atonement by another animal, accordingly this animal which is the surplus of the sin-offering must be left to die.');"><sup>8</sup></span> and the third must be left to pasture;<span class="x" onmousemove="('comment',' For it is not directly connected with the first animal.');"><sup>9</sup></span>