The plant-specific B3 superfamily encompasses well-characterized families, such as the auxin response factor (ARF) family and the LAV family, as well as less well understood families, such as RAV and REM.

All members of the B3 superfamily contain an ~ 110 amino acid region called the B3 domain. This domain was initially named because it was the third basic domain in the maize gene VIVIPAROUS1 (VP1). The first and second basic domains (B1 and B2) are specific to the VP1-like proteins, but genes that contain the B3 domain are widespread in plant genomes. The B3 domain of VP1 encodes a sequence-specific DNA binding activity.

Swaminathan K, Peterson K, Jack T.
The plant B3 superfamily.
Trends Plant Sci, 2008. 13(12): p. 647-55.
PMID: 18986826

Note: In plantTFDB, B3 family includes LAV family, REM family, and RAV family (those do not contain AP2 domain).