Hi Sale Mum
You should definitely appeal a near miss of 3 marks. You don't say what the pass mark is for Sale, but 3 marks must be a fraction of a percentage point.
Please read Etienne's Appeal Q & As (link at the top of the page). There is also quite a helpful information page about Appeals in Trafford here:
If your daughter is working to level 5 SATs already, you should have good all round academic evidence. CATs scores (if any) can also be very helpful, as are strong letters of support from the current Head and form teacher, plus good schoolbooks for the core subjects.
If you can accumulate all of that, your only remaining task is to explain "what went wrong on the day".
If you need help with any specific issues, please post back.
Edit: I have just looked through the published Trafford Admission information (which is really pretty impressive, I must say), and I would add the caveat that you should consider your Appeal in the light of the Admissions criteria. If your DD is very unlikely to gain a place on the published criteria, then you might not want to put yourself through the pain of an Appeal. If, however, you are in catchment, then I would advise you to go ahead.