Hierarchy query with 3 tables
I've never tried anything like that, but what if you UNION your three tables into one, and then do a "one-table hierarchical query" on the result of that?

WTIH alltabs as
    (Select 'T1' as src, Table1.ID, NULL AS ID_table1, NULL as ID_table2, Table1.name, Table1.size
    from Table1
    union
    Select 'T2' as src, Table2.ID, Table2.ID_table1, NULL as ID_table2, Table2.name, Table2.size
    from Table2
    union
    Select 'T3' as src, Table3.ID, Table3.ID_table1, Table3.ID_table2, Table3.name, Table3.size
    from Table3)
Select *
from alltabs
/*you can have an optional STARTING WITH clause here, but it's not clear from your question what that would be*/
connect by (id = id_table1 and (src = 'T1' or src = 'T2'))
        or (id = id_table2 AND (src = 'T2' or src = 'T3'))


(I'm assuming that the relationship is Table1 PARENT OF Table2 PARENT OF Table3).