Bob,
you are buying 3 years notional service which will give you 40 years service i.e. the maximum for pension entitlements. You therefore will be receiving a pension of half your final salary at retirement. If you don't believe you can live on half your salary then maybe you should look at AVCs, but it is up to you. I personally would be looking at eradicating debts (if any) before retirement, if I had spare cash.
You should talk to your HR department or Union to provide you with a list of those who provide AVCs within your organisation.
Regards,
Evening