Hi Jörg, I'm a little confused about what you're asking about - you are mentioning 3 parts, but I only see 2 parts in the model you attached, this one here:



And this one here:



Are those the ones you're trying to boolean union together?

You can run into probems with 2 parts that are just barely skimming each other with only overlapping surface area and not actually pushing through each other. If you have 2 things that are just skimming over the same surface area like that, they will need to very accurately coincide in order to be unioned. Your 2 pieces look like they are slightly different in shape, one piece has segmentation here:



While the other does not:



You might try moving the pieces a little bit so that they are pushing through each other a little bit instead of barely skimming, or you could try building the pieces with the same structure so that they would be more exactly touching one another instead of one maybe dipping slightly in and out of the other's surface area, or possibly a better method might be to build the whole body as one larger piece and then instead slice a groove in it.

It's not totally clear to me whether those 2 parts are actually one solid piece in the actual object with just a groove in that separating area or whether it's 2 separate pieces. If it's a groove you may want to model it more like that with the whole thing as one big piece and then cut out grooves.

- Michael