Welcome to Vodafone Community
We're sorry to hear you've had to send the handset for repair on three occasions. Please be assured that the repair team are doing their very best to get it back to you as soon as possible.
We hope that this will be the last time that you have to send it off.
We would be unable to comment on why the board has been replaced as we do not have the phone.
In regards to your question about the normality of the occurrence, it does not seem to be a common issue.
As long as the handset is sent for repair through Vodafone, repairs will not effect the warranty.