declare @cat table
( id int)

declare @com table
( id int)

declare @ut table
( id int)

insert into @cat
select id from tab_categorie where descrizione like '%' + @search + '%'

insert into @com
select id from tab_comuni where descrizione like '%' + @search + '%'  

insert into @ut
select id from tab_utenti where nominativo like '%' + @search + '%' or azienda like '%' + @search + '%'

select  u.nominativo, u.azienda, c.descrizione as comune, ca.descrizione as categoria
from  tab_utenti u
    inner join tab_comuni c
      on c.id=u.idcomune inner join
    tab_categorie ca
      on u.idcategoria=ca.id
where  u.id in (select * from @ut) or
    u.idcategoria in (select * from @cat) or
    u.idcomune in (select * from @com)