I have the following 3 boards that I would like to manufacture. when I export it into Gerber it consider it as one board. How to split it into 3 separates boards?
First, your PCB’s look atrocious. PCB design is more then drawing some tracks and filling some zones. At the moment you don’t have a decent GND plane on any of your PCB’s, and it would not require much work to put the tracks on one layer and using the other layer as a GND plane.
For your Gerbers. At the moment this is officially still “not supported”. The simplest workaround probably is to:
- Save the project.
- Delete two of the PCB’s.
- Create Gerbers of the remaining PCB.
- Revert to the full project.
- Repeat from step two onwards for the other two PCB’s.
I know. This is the thing I always stuck at! I just don’t know how to route a PCB… It looks too complicated… 
Thanks this worked great!
Yes, that’s pretty clear from your track layout. It’s a typical example of a beginner letting an autorouter do the work without even trying to put some manual effort into the design.
Yes, looking at the layout there’s not much that could be worse.
If you are willing to learn, you might try posting your design here (schematic and layout, in a new thread) and ask for feedback. That’s somewhat out of the focus of this forum, which is about using KiCad and not learning electronics. Still might be close enough to get valuable feedback.
Another option would be the beginner section of the eevblog forum https://www.eevblog.com/forum/beginners/
Mabe final note: Doing proper PCB layout is 90% about placing the components. Actual routing is simple in many cases.