SELECT tbl_a.id, tbl_a.name,
COALESCE( ( SELECT json_agg(tbl_b.*)
FROM tbl_b
WHERE tbl_b.a_id = tbl_a.id ), '{}'::json),
COALESCE( ( SELECT json_agg(tbl_c.*)
FROM tbl_c
WHERE tbl_c.a_id = tbl_a.id ), '{}'::json)
FROM tbl_a;