If you don't know them you can't declare them. If you've met the minimum requirement with the ones you can remember then why would it matter? The worst case scenario is that they'd ask at interview why you only sat three GCSEs and then you could explain but in my experience (4 interviews, 4 offers, 1 enrolment) GCSEs weren't mentioned, no-one was interested in anything other than my recent course (access) and my recent work experience. Applying as a mature student is different, no-one is really interested in what you did nearly a decade ago.
Or, put the subjects down that you did and just 'guess' the grade. I doubt it would make any difference to your application whether they were all As or all Es.
To give you an example of how relevant GCSEs are I applied for a course which usually asks for A at A'level chemistry. On my application I put that I got an E in GCSE chemistry years ago.