Trifunctional enzyme that catalyzes three distinct reactions as part of the 'de novo' inosine monophosphate biosynthetic pathway. {ECO:0000305|PubMed:12450384, ECO:0000305|PubMed:12755606, ECO:0000305|PubMed:20631005, ECO:0000305|PubMed:2183217}. This is the function of GART (phosphoribosylglycinamide formyltransferase, phosphoribosylglycinamide synthetase, phosphoribosylaminoimidazole synthetase, Ensembl gene identifier ENSG00000159131).