The elliptic curve Ek:y2=x3+k admits a natural 3‐isogeny ϕk:Ek→E−27k. We compute the average size of the ϕk‐Selmer group as k varies over the integers. Unlike previous results of Bhargava and Shankar on n‐Selmer groups of elliptic curves, we show that this average can be very sensitive to congruence conditions on k; this sensitivity can be precisely controlled by the Tamagawa numbers of Ek and E−27k. As a consequence, we prove that the average rank of the curves Ek, k∈Z, is less than 1.21 and over 23% (respectively, 41%) of the curves in this family have rank 0 (respectively, 3‐Selmer rank 1).