I have had something like this on a certificate from the GRO. It seems to be recording a correction.

In these scans from one of my certificates you can see 23 next to where Scoot has been crossed out and replaced by Scott. I think the dash above the 3 of 23 is just the tail from a letter on the record above. In the left margin you can see Twenty three written in full and the initials W.C. As the (local) Registrar is recorded as Wm Cross I think he must have made a correction and initialled it. When his copy reached the GRO they dutifully copied the correction in full.
On yours it looks like the number 3 was to record something being crossed out of the address and the squiggle by the oblique line may be Three with the Registrar's initials below. I agree with ireneb that what looks like a 7 next to the E of Esther is just part of the E with a bit faded away.
You could do as ireneb suggests and request another copy from the local office which would confirm this, if they are a helpful local office they will probably do a certified photocopy if you ask rather than them transcribing it which may lose this data. Alternatively send an email to the GRO at
col.admin@ons.gsi.gov.uk with GQ in the subject line so you don't get an auto reply (see
http://www.gro.gov.uk/gro/content/conta ... ection.asp for more information). Give them full details of the certificate including the application number, date you ordered and all the usual references (these will probably be on the coloured sheet that came with the certificate when they posted it to you). I have found them most helpful in the past and if they do send an answer which either confirms my thoughts or gives another answer please copy it back to this posting.