Transcriptional activator that specifically binds DNA sequence 5'-AGAAnnTTCT-3' known as heat shock promoter elements (HSE). Involved in heat stress response. Activated by DREB2A under heat stress. Homotrimer. Detected under heat stress condition. By heat stress. The hydrophobic-rich region (HR-A/B) corresponds to the oligomerization domain. AHA motifs are transcriptional activator elements. Exhibits temperature-dependent phosphorylation. Plants overexpressing HSFA3 show increased thermotolerance. Belongs to the HSF family. Class A subfamily.