Looking on Google (e.g. here:
http://www.brettmartin.com/~/media/Files/Daylight-Systems-Document-Library/Technical-Documents/BBA-4114i2---GRP.pdf ) this looks like a combination of specifications - SAB is a rating to the BS roof test ("S" = "Sloped", "AB" indicating relative time to penetration and spread of flame under the old BS 476: Part 3 test). It looks like the "3" tacked on the end indicates Class 3 to BS 476: Part 7. This would be relevant to rooflights, where fire performance from both above (the "SAB") and below (the Class "3") are controlled.
BS 476: Part 3 is a test for roofing
systems (not individual products or components), whilst BS 476-7 is a product test standard covering spread-of-flame performance. I'm not sure why someone would be specifying a roofing test for a wall construction? In any case, in answer to your question Class 3 to BS 476-7 is nowhere near to Class 0 - as a guide, Class 3 is generally reckoned to be about the same level of performance as untreated timber.
Wise to be cautious - generally speaking it's not supportable to convert ratings between reaction-to-fire test standards as they tend to address completely different aspects of fire performance. About the only comparisons you can draw with confidence are those published in guidance (e.g. the tables comparing BS and BS EN tests in the AD-B).