Chromatin reader that recognizes and binds acetylated histones, thereby controlling gene expression and remodeling chromatin structures (PubMed:18406326, PubMed:22464331, PubMed:27105114, PubMed:32895492). Recruits transcription factors and coactivators to target gene sites, and activates RNA polymerase II machinery for transcriptional elongation (PubMed:29567837, PubMed:32895492). In vitro, binds acetylated lysine residues on the N-terminus of histone H2A, H2B, H3 and H4 (PubMed:18406326). Involved in endoderm differentiation via its association with long non-coding RNA (lncRNA) DIGIT: BRD3 undergoes liquid-liquid phase separation upon binding to lncRNA DIGIT, promoting binding to histone H3 acetylated at 'Lys-18' (H3K18ac) to induce endoderm gene expression (PubMed:32895492). Also binds non-histones acetylated proteins, such as GATA1 and GATA2: regulates transcription by promoting the binding of the transcription factor GATA1 to its targets (By similarity). {ECO:0000250|UniProtKB:Q8K2F0, ECO:0000269|PubMed:18406326, ECO:0000269|PubMed:22464331, ECO:0000269|PubMed:27105114, ECO:0000269|PubMed:29567837, ECO:0000269|PubMed:32895492}. This is the function of BRD3 (bromodomain containing 3, ENSG00000169925).