Acts as a mitochondrial iron-sulfur (Fe-S) cluster assembly factor that facilitates [4Fe-4S] cluster insertion into a subset of mitochondrial proteins such as lipoyl synthase (LS) and succinate dehydrogenase (SDH) (PubMed:27532773, PubMed:27532772). Required during the last step of iron-sulfur protein assembly when the iron-sulfur cluster is inserted into the target protein (PubMed:27532772). Acts together with NFU1, later than BOL1 and GRX5 in the [4Fe-4S] cluster insertion process (PubMed:27532773). Not required for [2Fe-2S] cluster insertion into mitochondrial proteins (PubMed:27532772). Interacts with NFU1 (PubMed:27532773). Increases frequency of mitochondrial genome loss (PubMed:19300474). Cells show a slight decrease of succinate dehydrogenase (SDH) (PubMed:27532772). Cells lacking BOL1 and BOL3 display defects in a subset of mitochondrial [4Fe-4S] enzymes (PubMed:27532773, PubMed:27532772). Present with 259 molecules/cell in log phase SD medium. Belongs to the BolA/IbaG family.