Nope, I shouldn't think so, the 3 years starts from the date of first registration. If it was a Renault or similar, you might be in luck, but I've never heard anything other than bad stories about Chev Uk and their discretion on warranties. Doesn't sound like sharp practice at all, pre reg is pre reg. I've owned a number of pre reg cars in my time and understood that this was the trade off for the lower pricing.
That said, it's a total bugger!