Aldegunde,
"I work only three hours a day",
and
"I work only a few hours a day", are perfectly acceptable.
"Only" goes immediately before, or after the words that you are modifying. Strictly speaking, "I only work" means that I do nothing else.
"I only play Badminton on a Monday" means that I play Badminton and nothing else on a Monday. Badminton! Badminton! Badminton! Badminton! Badminton! All day long!
"I play Badminton only on a Monday" or "I play Badminton on a Monday only", mean that Monday is the only day on which I play Badminton.
Beware! Many English speakers do not obey this Rule!
I apologise for �correcting� you, but "a few" does not mean "several", it means "not many" or "a small number".
"We had a party last night and I invited 20 friends but only a few came, a small number - 6 in total".
I hope that this helps you.
Best wishes.
Les