Three Possibilities

I am looking to trace my great grandmother's birth certificate to hopefully
find out her parents names from the certificate to dig back further.

From census records, I have found an approximate year of birth and place for
her. Now, the GRO is bringing back three possibilities.

Anybody offer any advice on what I can do to work out which one is right? I
just don't wanna fork out on three certificates. I am assuming I can look at
marriage certs as I have her husband's name and then tie together the DOBs
on the marriage cert and the birth cert but I need the certs to do this
don't I? Is there a cheaper way?

Eve McLaughlin...

You should be working back anyway from what you know to what you don't.
A marriage certificate will give you the names of the two fathers and,
after c 1870, an exact age. This should enable you to check for the
person in the appropritae census (every ten years back from 1901) and
this will give you the place of birth as well as an approximate date.
So by the time you get to needing your gt grandmother's birth
certificate you should already know
Her age
Her father's name
Her stated birthplace
This should lead you to the correct entry in the GRO indexes - unless,
of course, she is Mary Smith born 'London' or Jane Jones born 'Wales'.
But even then, you have the father's name, so any request for a birth
certificate can go in as 'only if father's name George or reasonable
variant'. Thislimits you (and your expenditure) to the birth
certificate which matches the data.
(never be led into using the occupation of the father (from marriage
cert) as the only acceptable occupation on the birth certificate -
things can change iin 20-30 years.)