What do you want to for a job? - the cert is what you need to be an assistant. If you intend to be a group leader in childcare at any point, you will have to do the diploma. The bachelor qualification you are thinking of doing - what do you hope to do from this?
If you want to do something like child protection, and don't intend to work in childcare, you could bypass all of the courses including cert 3 and just start the bachelor as you don't get many credits from tafe courses for uni degrees.