The three monthly blood tests give you an average of what has happened over that period, but they don't tell you how that average is made up - e.g. are your glucose levels fairly steady, or sometimes going too high or too low. So buying your own meter and doing regular testing will show you which foods can do nasties to your glucose levels, and this is definitely the best investment you can make diabetes wise.
If you're in the UK the SD Codefree is a good meter to buy as the test strips are very reasonably priced. You can buy the meters from eBay or (slightly dearer when I last looked) from Amazon UK. Test strips are about £6.99 from either site.
I bought my meter before I found this forum and heard about the Codefree, but even so I believe it's been worth every penny!
Robbity