Assuming we can have more than three pokemon at a time...

Electrode vs. Mega-gyarados: electrode wins!
Mega-pidgeot vs. vileplume: M-Pidgeot wins!
Rhydon vs. farfetch'd: rhydon wins!

Adding Mega-Pidgeot.

Team holding gym: Valor
Pokemon: M-Gyarados, Vileplume, Farfetch'd, M-Pidgeot