2003 chevy venture car will not start somtime then all of sudden it will start may take one day or week before able to start .
Have replaced battery new starter had emmission system tested ,oxygen sensor
Had it towed to garage 3 times each time it starts after tow . Mechanic says he can not find problem when it is not doing it for him .

The shop may need to do a voltage drop test.
Did they check for any stored diagnostic trouble codes.. or did they just turn the key and it started?

Sounds like intermittent Starter Relay failure. Try replacing the relay, located in the under hood fuse/relay box (AutoZone Part No. 19307, $12.99).