Hi Loobie,
hopefully I'll get this right as I'm a Registrar! for unmarried parents, if you both attend to register baby together, dad's name will appear on the certificate. if mum goes alone and dad's details are left blank, they can be added by re-registration at any time - this means you have to get a form from your local register office (one for each child), sign it to declare that you both agree he is the natural father, and then they will give you an appointment where you both go in to reregister the children - this needs you too, so he can't do it alone. (if you want a new certificate, get it at this time, as because it will be a 'new' entry it will only cost 3.50 and not 7 pounds (can't find the pound sign on this stupid machine!))
however - he can go to a solicitor for advice and take it through the courts to get parental responsibility. we have one at the moment where dad has written in to say that he can prove he's the dad as they've had a DNA test done, but it's still all in the hands of the court, and it doesn't necessarily mean that his name will eventually be on the birth certifciate, but he may get a document from the court to prove he is the natural parent and that he has parental responsibility.
so if you DO want his name added for all 3, you just need to get in touch with the local register office for the relevant forms. for any children born before 1 December 2003, being on the birth cert doesn't give him any additional rights, he would still need to go to court to obtain equal parental rights. After 1 December 2003, if he signed the register then he has equal parental rights with you, exactly as if he was married to you at the time of birth.
Phew! hope that makes sense - let me know if you need anything clarifying!