That's not how qualifications work, a CertHE is equal to the first year of undergraduate study, and a DipHE is equivalent to the first two years of undergraduate study. You still won't have studied at a level to complete the equivalent of the final year of university study. Both qualifications are lower than a bachelors qualification. You may be able to apply for credit transfer with the OU and have some of the credits counted towards what would be the third year of a full time degree, depending on what level of modules you were taking then, but this is not guaranteed.
However, generally you get full time funding = length of current course + 1 year - years of prior study in HE. So in your case for a 3 year degree it would be 3 + 1 -2 = 2 years as noted. Note though funding is calculated backwards from the end of the degree, so if you don't have funding remaining to complete the full course you will need to self fund the first year(s) in the new degree. So I don't think your plan is going to work as expected - as once you transfer to the new course, you would need to self fund tuition fees for the first year there. You usually would still be able to apply for a maintenance loan for the years you are self funding tuition fees however.
In terms of funding otherwise, depending on why you left the first time you may be able to apply for an additional year of funding under compelling personal circumstances (CPR) - examples of things you could apply under include health issues, mental health challenges, bereavements, etc. The important thing though is you'll need documents for evidence for these (e.g. discharge summaries, medical reports, letters from GP/psychiatrist/therapist, death certificates, etc). Also if you are otherwise able to make arrangements to self fund the tuition fees for that first year in the new course then you would be funded as per usual for the remainder (provided you don't need to retake a year and don't change course again).
Also note that you can be funded still for a part-time course if the qualification you are studying towards isn't an equivalent or lower level qualification (ELQ) to one you studied before, as the funding models for part-time and full time study are separate. Also note that all study at the Open University is funded through the part-time model, regardless of study intensity - so you could apply for credit transfer to the OU and complete your degree there potentially (however normally you can't get a maintenance loan when studying at the OU). So that may also be an option.