Catalytic subunit of the PI3K complex that mediates formation of phosphatidylinositol 3-phosphate; different complex forms are believed to play a role in multiple membrane trafficking pathways: PI3KC3-C1 is involved in initiation of autophagosomes and PI3KC3-C2 in maturation of autophagosomes and endocytosis (PubMed:14617358, PubMed:33637724, PubMed:7628435, PubMed:33993848). As part of PI3KC3- C1, promotes endoplasmic reticulum membrane curvature formation prior to vesicle budding (PubMed:32690950). Involved in regulation of degradative endocytic trafficking and required for the abscission step in cytokinesis, probably in the context of PI3KC3-C2 (PubMed:20208530, PubMed:20643123). Involved in the transport of lysosomal enzyme precursors to lysosomes (By similarity). Required for transport from early to late endosomes (By similarity). {ECO:0000250|UniProtKB:O88763, ECO:0000269|PubMed:14617358, ECO:0000269|PubMed:20208530, ECO:0000269|PubMed:20643123, ECO:0000269|PubMed:32690950, ECO:0000269|PubMed:33637724, ECO:0000269|PubMed:33993848, ECO:0000269|PubMed:7628435}. (Microbial infection) Kinase activity is required for SARS coronavirus-2/SARS-CoV-2 replication. . This is the function of Ensembl gene identifier ENSG00000078142 (PIK3C3, phosphatidylinositol 3-kinase catalytic subunit type 3).