Why are my tomatoe plants leaves still turning yellow?

Have you had successful tomatoes plants in this location before? Is there a walnut tree anywhere nearby?

Why just leave three leaves on a tomatoe plant?

Sounds like an iron deficiency. Products like Ironite may help with this.

OK Why did you ask if there was a walnut tree near by??

Walnut trees and their fruit, tend to poison other plants around them. That is why bellanger asked.

Tick

Mine were turning yello and I fertilized with a water soluble fertilizer and they greened up quickly.