Looking them up in dictionaries gave me this:
häufig – frequent, common
gewöhnlich – ordinary, common, habitual, normal, usual
üblich – common, usual, ordinary, normal.
I looked online for the difference but could not find anything, only examples in dictionaries.
I was told you cannot say “Es war häufig.”, but “Es war üblich” is possible. Is that right? Any particular reason why?
I know that gewöhnlich can be slightly negative, when it means nothing special, nothing worth mentioning (“Sie sieht recht gewöhnlich aus.”). Are they interchangeable in all other cases? For example:
a) Sie haben es häufig/üblich/gewöhnlich vergessen.
b) Das ist ein häufiger/üblicher/gewöhnlicher Fehler.
c) Sie sind wie üblich/gewöhnlich/häufig gekommen.Are there any rules when to use each or should I simply memorize as much as I can, play by ear when I try to use them and hope for the best?