Multifunctional protein with various roles in different cellular compartments. May act in a redox sensitive manner. Associates with chromatin and binds DNA with a preference to non-canonical DNA structures such as single-stranded DNA. Can bent DNA and enhance DNA flexibility by looping thus providing a mechanism to promote activities on various gene promoters (By similarity). Binds to the delta-1 crystallin/ASL1 enhancer (PubMed:7904558). Proposed to be involved in the innate immune response to nucleic acids by acting as a cytoplasmic promiscuous immunogenic DNA/RNA sensor (By similarity). Found in newly hatched chick liver and decreases during postnatal development. Reduction/oxidation of cysteine residues Cys-23, Cys-45 and Cys-104 and a possible intramolecular disulfide bond involving Cys-23 and Cys-45 give rise to different redox forms with specific functional activities in various cellular compartments: 1- fully reduced HMGB3 (HMGB3C23hC45hC104h), 2- disulfide HMGB3 (HMGB3C23-C45C104h) and 3- sulfonyl HMGB3 (HMGB3C23soC45soC104so). Belongs to the HMGB family.