Welcome to the Community Forum and thanks for your post. I'm sorry to hear that you are having problems with your Samsung J3 connecting via your VW car Bluetooth system.
Is the mobile able to connect to other Bluetooth devices (wireless speakers or headphones)?
Are other mobile handsets able to connect via Bluetooth to your car's system?
In order to find out what's causing the problem, please try the following when in your car:
From your Home screen, touch Apps
Go to Settings > Bluetooth
Move the slider to turn Bluetooth on
Your handset should now scan for available Bluetooth devices.
Do you see the Bluetooth option for your car's system?
If you do, just click on this and you should be able to connect. Is this working now?
If you don't see your car's system then you may need to consult your car's owner manual or car dealer for help with the on-board Bluetooth system, especially if no other mobile devices are able to connect.
I had proceeded as you suggested the problem was that although the car showed the phone as connected, when I called out it either connected then cut out or connected and the receiver could only hear a distorted voice. VW checked it out and it worked fine with their mobiles so suggested it must be my phone. When I checked my blue tooth setting on the phone it showed it as connected for media but not calling and messaging. when I tried to switch these on they kept dropping out. So I went to reset setting on the phone and although I had to re install ring tones etc no great problem. It now seems to be ok, although I check to make sure that 'call audio' is switched on when I get in the car. Fingers crossed