Phosphoinositide-binding protein required for multivesicular body formation. Specifically binds phosphatidylinositol 3-phosphate (PtdIns(P3)). Can also bind phosphatidylinositol 4-phosphate (PtdIns(P4)), phosphatidylinositol 5-phosphate (PtdIns(P5)) and phosphatidylinositol 3,5-biphosphate (PtdIns(3,5)P2) (By similarity). Plays a role in protein transport between cellular compartments. Together with RAB7A facilitates endosome membrane association of the retromer cargo-selective subcomplex (CSC/VPS). May in part act as component of the SNX3-retromer complex which mediates the retrograde endosome-to-TGN transport of WLS distinct from the SNX-BAR retromer pathway (PubMed:21725319, PubMed:24344282, PubMed:30213940). Promotes stability and cell surface expression of epithelial sodium channel (ENAC) subunits SCNN1A and SCNN1G (By similarity). Not involved in EGFR degradation. Involved in the regulation of phagocytosis in dendritic cells possibly by regulating EEA1 recruitment to the nascent phagosomes (PubMed:23237080). Involved in iron homeostasis through regulation of endocytic recycling of the transferrin receptor TFRC presumably by delivering the transferrin:transferrin receptor complex to recycling endosomes; the function may involve the CSC retromer subcomplex (By similarity). In the case of Salmonella enterica infection plays arole in maturation of the Salmonella-containing vacuole (SCV) and promotes recruitment of LAMP1 to SCVs (PubMed:20482551). Interacts with VPS26A, VPS29 and VPS35; the interaction with VPS35 is direct. The association with the retromer CSC subcomplex subunits is proposed to represent a functional distinct retromer variant described as SNX3-retromer complex (PubMed:21725319, PubMed:24344282, PubMed:30213940). Interacts with USP10 and SCNN1A (By similarity). Interacts with TRFC (By similarity). Interacts with SNX8; 2 molecules of SNX8 seems to associate with one molecule of SNX3 (PubMed:24866125). Interacts with PTPRU (PubMed:17622474). Interacts with MON2 and DOP1B. Colocalizes to clathrin-coated endosomal vesicles morphologically distinct from retromer-decorated non-branched endosomal tubule structures (PubMed:21725319) Colocalizes with EEA1 on nascent phagosomes in dendritic cells but competes with EEA1 for binding to phagosomal membrane (PubMed:23237080). In the case of Salmonella enterica infection localizes to Salmonella-containing vacuoles (SCVs) from which SNX3-containing tubules form 30-60 min after infection (PubMed:20482551). The PX domain mediates specific binding to phosphatidylinositol 3-phosphate (PtdIns(P3)). Ubiquitinated, leading to its proteasomal degradation. Deubiquitinated by USP10 (By similarity). A chromosomal aberration involving SNX3 has been found in patients with syndromic microphthalmia. Translocation t(6;13)(q21;q12). Belongs to the sorting nexin family.