Transcriptional modulator activated by TGF-beta (transforming growth factor) and activin type 1 receptor kinase. SMAD3 is a receptor-regulated SMAD (R-SMAD) (By similarity). Interacts with SARA (SMAD anchor for receptor activation); form trimers with another SMAD3 and the co-SMAD SMAD4. In the cytoplasm in the absence of ligand. Migration to the nucleus when complexed with SMAD4. The MH2 domain is sufficient to carry protein nuclear export. Phosphorylated on serine by TGF-beta and activin type 1 receptor kinases. Belongs to the dwarfin/SMAD family.