Lipid phosphatase that has high specificity for phosphatidylinositol 3-phosphate and has no activity with phosphatidylinositol 4-phosphate, phosphatidylinositol (4,5)-bisphosphate and phosphatidylinositol (3,4,5)-trisphosphate (PubMed:11733541, PubMed:27018598). Activity with phosphatidylinositol (3,5)-bisphosphate is controversial; it has been shown by PubMed:27018598, while PubMed:11733541 find no activity with this substrate. a 1,2-diacyl-sn-glycero-3-phospho-(1D-myo-inositol-3,5-bisphosphate) + H2O = a 1,2-diacyl-sn-glycero-3-phospho-(1D-myo-inositol-5-phosphate) + phosphate a 1,2-diacyl-sn-glycero-3-phospho-(1D-myo-inositol-3-phosphate) + H2O = a 1,2-diacyl-sn-glycero-3-phospho-(1D-myo-inositol) + phosphate 1,2-dioctanoyl-sn-glycero-3-phospho-(1-D-myo-inositol-3-phosphate) + H2O = 1,2-dioctanoyl-sn-glycero-3-phospho-(1D-myo-inositol) + phosphate Homodimer. The C-terminal region is required for dimerization. Belongs to the protein-tyrosine phosphatase family. Non-receptor class myotubularin subfamily.