ne pas casser trois pattes à un canard

to be nothing out of the ordinary

Whatever is being described is perfectly adequate and completely forgettable. It is what people reach for about a meal or a support act that does the job and no more, with no great unkindness intended.

Son dernier film ne casse pas trois pattes à un canard, mais on passe un bon moment.

His latest film is nothing special, but it's an enjoyable watch.

The usual explanation is arithmetic: a duck has two legs, so breaking three of them would be quite a feat, which is the joke. A less common theory has 'canard' once meaning a broken-down horse, and says only a reckless rider would risk damaging one.