The following will join the dep_type values from t1 with only the records from t2. Your data sample does not have any matching ex_line
values common to t1 and t2. Is this true of the entire dataset? It if isn't true for the entire dataset, then @OldProgrammer is right - the join will not return the exact number of records in t1.
SELECT
t1.position,
t1.ss_id,
t2.ex_line,
t1.dep_Typ,
t1.num
FROM
t1
INNER JOIN
t2
ON t1.position = t2.position
AND t1.ss_id = t2.ss_id
INNER JOIN
t3
ON t1.ss_id = t3.ss_id
AND t1.value = t3.value
AND t2.vech_num = t3.vech_num
;