I've just connected Sonar x1 Expanded to my A-3-- Pro keyboard, but every time I depress a key on the keyboard with any soft synth instrument the note keeps triggering over and over again until I release the key. I am not getting the normal single note and falloff, but each note sounds multiple times like C C C C C C C C with fast even beats until I release the key. There is probably a simple explanation for this, but an extensive search on Google has failed to reveal any solution. Can anyone help me please?

How have you connected the A3?
 
If you've got Midi in and out connected then it's possible you're getting a "loop back".
 
Try disconnecting the Midi in on your A3.
 
Also, see if you've got Local on disabled on the keyboard

Do you mean the A-300Pro? If so, once the drivers are installed Sonar will show 3 midi inputs. One is from the actual keyboard (A-pro 1), one is from the controller function of the keyboard (A-pro 2), and the other is the external midi input on the side of the keyboard (A-Pro Midi in).
 
In Sonar, make sure you have chosen only the keyboard input for your midi track(A-Pro 1 Midi omni), and that the output of that track is set to the synth you are using for audio and not looping back to the A-300Pro
 
If your keyboard is not an A-300Pro, ignore all of that:)

Thanks very much for your suggestions.
I have the A-300 PRO connected to the PC only through the USB, which also powers the A-300Pro.
I have taken your advice and ticked only A-PRO 1 in Sonar's MIDI Devices inputs, and left the other inputs and all Outputs blank (unchecked). Nothing I have tried has fixed the problem of the notes repeated triggering.
I don't know how to disable Local On on the A-Pro keyboard, but I understand it can be done in Sonar - how?
 
 

I don't believe you can disable Local On on your keyboard through Sonar.  You should download the manual from Roland for your keyboard and it will give you instructions on how do that and other things on your keyboard.
 
To find out if its a Sonar thing or a keyboard thing, you could record holding the C note down.  If you see 1 long continuous midi note in sonar, then it is a sonar problem.  If you see multiple short notes, then the keyboard is sending multiple midi notes to Sonar and its a keyboard problem.
 
Also, what patch are you using?  I would diagnose this with a basic piano patch.  Some patches actually will sound like repeated notes when a key is held down.

Is the arpeggiator on?
 
Dan
 

You don't need to disable local control. In fact it you can't - the A300 is just a controller. It has no onboard sounds.
 
Have you selected just midi-1 omni on the midi tracks input drop down menu? What is the Midi out of the track set to? Insert any vst synth and set it to that. You need to ensure there is no loopback happening.
 
As Dan suggested check that the arpegiator is not active on the midi track you are recording on. 
 
When you record, do the multiple notes appear in the resulting clip?
 
You may also want to try the other A-pro inputs one at a time just in case your system has assigned the midi ports differently to the default.
 
 

I discovered the reason I was getting multiple triggers. I checked in Preferences/ Devices and unchecked the PC's Realtek HD Audio input and output, leaving just the Fast Track audio interface as the main in/out.  This fixed the problem and I am now getting clean notes.  Thanks, everyone for your input and suggestions.