How to say, "I misunderstood." - svp

Question

If I wanted to say, "I misunderstood," would any one of the following be acceptable?Je me suis mépris. J'ai mal compris.Je me suis trompé.

Accepted Answer

Mark, 
‘ Je me suis trompé ‘ and ‘ Je me suis mépris ‘ are not quite the same as ‘ j’ai mal compris ‘.
‘J’ai mal compris ‘  translates as the relatively benign/neutral ( in terms of blame/cause )  -  ‘ I misunderstood ‘. You are not saying why you misunderstood, or that it was your ‘ fault ‘.
The other 2 are more ‘ mea-culpa ‘ statements - ‘ I made a mistake/ I was wrong ‘.

Answer

Bonjour Mark,
Maarten is correct about all 3 expressions (and their meaning)!
Just one little pointer: "je me suis mépris" is not common at all ; using the verb "se méprendre" is extremely rare and is very formal (you might encounter this verb is literature but not in an everyday context/conversation).
I hope this is helpful.
Bonne journée !