We are playing a “flat back 3” defense in our 3-4-3 triple diamond formation. What this means is we don’t have a sweeper who defends against thru balls and acts as a safety valve when one of our backs gets beat by an opponent. A "flat back 3" defense is somewhat of a misnomer - it's more of a jagged line than a straight line across the field.
The "flat back 3" works well if our defense (left, center, and right) forms a cover line. Some basics regarding a
cover line are highlighted in the article entitled “Slide and Slant” (see right margin - click and review). A cover line starts when the defender closest to the ball challenges for the ball (click on the soccer field to enlarge).
In figure #1, our center back makes the challenge while her teammates form a cover line in the shape of a pyramid. Notice the left and right defenders are both ball side and goal side (click “Mark Up – Ball Side – Goal Side” in right margin to review concepts).
In figure #2, the right back makes the challenge while the center and left back form a different shape cover line. Again, both supporting defenders are ball side and goal side. Obviously, if the ball was on the other side of the field, the left back would be making the challenge while the center 
and right backs would form a similar cover line as in figure #2.
In summary – a “flat back 3” defense is anything but flat. If it truly gets flat, we probably get burned (click pic on left). A cover line stays flexible (not flat) backing up the teammate who is challenging for the ball.
Note - The illustration does not show our 4 midfielders. However, it's important with a flat back 3 that our wing midfielder farthest from the ball comes back to defend the far post. She (in essence) becomes our 4th defender on the cover line.