If Competitor #3 is mimicking Competitor #1 — be different and stand out more. They have kinda made it easier for you because copying the competitor’s main images does not create any benefit to the listing, in fact, it just merges your product into the background like everyone else.
As Regan mentioned below you should produce a main image that stands out from ALL others, so that includes standing out from competitor #1 AND competitor #3 and others on page one. Your aim to get better visibility than others on page one.
We cover this in Module 08: Listing Optimization for Conversion – sometimes it’s as simple as adding a different stand-out color; changing the direction of your product (if all sellers have a dog in the image facing the left, your’s will face right toward your bullets; if most sellers have a large black labrador, you might have a tan labrador or different, but popular large dog. There are so many ways to be different but you never want to look the same.