Any Mendelian disease in which the cause of the disease is a mutation in the TRAF3 gene. TRAF3 haploinsufficiency caused by heterozygous loss of function (null) variants presents as an immune dysregulation syndrome of recurrent bacterial infections, autoimmunity, systemic inflammation, B cell lymphoproliferation, and hypergammaglobulinemia. The disease is TRAF3 haploinsufficiency (Monarch Disease Ontology id MONDO_0100513).