The Class 3 has morphed into the current Class 2. Yes, you can fly using the medical declaration if you wish. At the moment this is for a limited period only. What you knew as an aeroplane such as a PA28 or C150/!152/172 etc is currently no longer simply an aeroplane (sic) but a Part-121 type. Soon, I'm sure, this idiocy will be scrapped, it is left over from EASA, and by the time you complete the process may not affect you, If not changed Part-21 types will not be flyable using a Medical Declaration only non-part 21 types. A Medical Declaration has many restrictions to consider and may only be used in UK airspace.