Lipid phosphatase that specifically dephosphorylates the D-3 position of phosphatidylinositol 3-phosphate, generating phosphatidylinositol (PubMed:11733541, PubMed:27018598). Could also dephosphorylate phosphatidylinositol 3,5-bisphosphate to produce phosphatidylinositol 5-phosphate (PubMed:27018598). . This is the function of MTMR1 (myotubularin related protein 1, ENSG00000063601).