DNA-binding protein that specifically binds heat shock promoter elements (HSE) and activates transcription. HSF3 binds DNA constitutively only when the C-terminal region is deleted. Homotrimer. Cytoplasmic during normal growth and moves to the nucleus upon activation. Expressed in most tissues. High levels are found in erythrocytes and low levels in liver. Expressed during development. Belongs to the HSF family.