At 3 inches in length, they are not sexually mature. Likely just habits of the turtles.
My RES's dig constantly, even before they were sexually mature. Also, you will see them begin courting when they become mature. The male will get on top of the female, and it's quite noticeable when it happens. With RES's, the male has a flatter shell and is lighter in color. The female is darker in color and has a more convex shell. Any chance you can get piccies?