t'étais pas loin, manquait un regroupement
select a.IP_PANNE,a.LIBELLE,sum(a.nbr) from
(
SELECT p.ID_PANNE, p.LIBELLE, count(E.PANNE_ID1) as nbr
FROM PANNE as P left outer join ENVOI as E ON
p.id_panne= e.panne_id1
Where p.ID_PANNE <> 1
Group by p.ID_PANNE, p.LIBELLE
union
SELECT p.ID_PANNE, p.LIBELLE, count(E.PANNE_ID2) as nbr
FROM PANNE as P left outer join ENVOI as E ON
p.id_panne= e.panne_id2
Where p.ID_PANNE <> 1
Group by p.ID_PANNE, p.LIBELLE
union
SELECT p.ID_PANNE, p.LIBELLE, count(E.PANNE_ID3) as nbr
FROM PANNE as P left outer join ENVOI as E ON
p.id_panne= e.panne_id3
Where p.ID_PANNE <> 1
Group by p.ID_PANNE, p.LIBELLE ) a
group by a.IP_PANNE,a.LIBELLE