I think a crusade has to be neutral or allied to help you. I guess the thinking is that they too are occupying the province. If I'm not mistaken they were actually helping defend from a another crusade though that ones was deflated severly after trying to get into another province of mine.

Other than said example I think I've only had it once or twice where 3 different armies on the field, two of whom weren't allies. Usually the third force are rebels. It was more common in Shogun.