sorry i forgot you cant get 3rd all the time - so when you drain the pan - it takes exactly 3.75 litres to fill it to proper level or 4quarts u.s. thats how ive done every trans fluid change on a th350 ( double checking when hot of course ) and its never failed me you could try a drain and refill but it sounds like the 3rd gear clutches are gone which is normally how they end up going in for rebuild .
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