What's the difference between 3G-A SDI and 3G-B SDI?
Wikipedia does mention it - with a reference link included:


  Within this standard there are three formats known as Level A, Level B
  Dual Link (B-DL) and Level B Dual Stream (B-DS). The Level A format is
  the direct mapping of uncompressed 1080p (up to 60 fps) video into a
  serial digital interface at the nominal 3 Gbit/s. The Level B-DL
  format is the mapping of dual-link HD-SDI/SMPTE 372M (i.e.: 1080p up
  to 60 fps) in a single serial digital interface at the nominal 3
  Gbit/s. The Level B-DS format is the dual-stream carriage of two
  independent HD-SDI/SMPTE 292M signals (720p up to 60 fps or
  1080i/1080p up to 30 fps) in a single serial digital interface at the
  nominal 3 Gbit/s


The PDF reference has the descriptions starting on pg. 17.