HertogJan, you should tell us what mods you're using (although I know this answer quite well).
Optical Targeting Correction puts these range marks onto the A-Scope radar screen to correct the stock games attempt at having nothing more than "eye candy" for radar use. Using the stock game, the A-Scope range ruler has the ranges wrong. You'll not come close to getting a range estimate using the stock A-Scope scale. To correct it, the three range lines you're asking about were "added" to the scope with the OTC mod. Depending on which radar power distance you select (with the switch below the screen), you will read the "spike" return at one of the three different range scales on the scope.
The three different "ranges" for the A-Scope radar are 4,000 yards, 32,000 yards, and 60,000 yards. The 60,000 yard distance is read off the top line, the 32,000 yard scale is in the center, the 4,000 yards distance is read on the lower line.
It's impossible to remove the stock scale line at the very bottom of the screen, so ignore it (although with it, you can see just how much of an error it gives in range to a target compared to OTC's correction).
With the above return "spike", if the range setting were at the 4000 yard distance, the target is exactly 3,000 yards away (when you read the lower 4,000 yard scale). If your radar range setting were on the 32,000 yard distance (the medium power setting), you would read the figure off the center line of the "spike". The target would be calculated to be 24,000 yards away.
In the below image, the A-Scope uses a "Bearing Meter" (placed on the wall, above the scope screen) to display a targets position in relationship to the sub. Like the scope views, OTC allows you to zoom-in the view to get a better view of the device.
Sorry for the image clarity (I simply in-larged the previous image), this close-up view shows the "arrow" of the Target Bearing is pointing at 10 degrees. The target "spike" of the A-Scope screen is 10 degrees off the Starboard bow of the sub.
The below image describes the parts to the "Bearing Meter". The "arrow" (or Radar Indicator) rotates with the radar antenna, giving a relative bearing of the target when compared to the outer dial. The subs true heading (the inner dial) rotates with the subs direction of travel as compared to true north (at the top of the outer dial).
Since you're using OTC, please read the Documentation that's included with the mod. The manual called "Using Radar" should be of assistance.