Just join third table
SELECT
mg.go_id,
mm.mirna ,
g.go_id ,
g.category,
g.evidence,
g.term ,
mg.MSU7_LOC
FROM mirna_mrna mm
inner join mrna_go mg on mg.MSU7_LOC = mm.MSU7_LOC
inner join gene_ontology g on mg.go_id = g.go_id
where mg.go_id in ('GO:0009058') ;