The challenge was short and the target kills were reached quickly. Once the target number of camps are destroyed then number killed doesn't increase. The number shown at the top of the screen is the number killed in that stage of the challenge, so it means you killed 3 in the 3rd final stage.
Also, read my latest post in this thread - the challenge is broken! https://forums.ea.com/en/commandandconquer/discussion/249971/infected-challenge-where-did-the-bonus-go#latest