前足	355682
前脚	59317
前肢	29600
Koj: まえ‐あし【前足・前脚・前肢】  (Daijr/s similar)

It doesn't because the まえあし reading is common to them all. It's a bit difficult to determine the frequency of readings of compounds but Google hits seem to indicate that 前脚 and 前肢 are read まえあし more often than ぜんし or ぜんきゃく.
It's an example of JMdict's rather strong line on such merges.

Does this break 2/3 rule? or is it ok because of the intermediate steps

前足(まえあし) ⇔ 前肢(まえあし) ⇔ 前肢(ぜんし)

前足(まえあし) ⇔ 前脚(まえあし) ⇔ 前脚(ぜんきゃく)