select,
 f.*,
 fp.*
from
 forums f
inner join
 forum_permissions fp on f.forum_id = fp.forum_id
where
 f.parent_id = -1 and f.forum_view = 1 and fp.group_id in (1,2,3)
group by
 f.forum_id;

-- more readable but i've assumed group_id is a field in fp as your orginal code doesnt state


select,
 f.*,
 fp.*
from
 forums f
left outer join
 forum_permissions fp on f.forum_id = fp.forum_id
where
 f.parent_id = -1 and f.forum_view = 1 and fp.group_id in (1,2,3)
group by
 f.forum_id;