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Thanks!
Then I will go to the same shop, just to be sure.
As for the caves, I guess it's best to buy for example one medium cave in a different shape (unfortunately the small caves only come in one shape) and some small bamboo caves, so they will have some different shapes, materials and sizes to choose from.

Today I've bought two more P. maccus from the same store as before. I tried really hard to pick two females, but it looks like I'm not very good at buying the right fish, because I'm afraid I ended up with an all male group.

Here are some pictures:

New arrival 1:


New arrival 2:


My existing resident:


I also took some pictures from above:

Existing P. maccus:


New arrival 1:


New arrival 2:


If I look at the pictures that I took from above, it looks like they all have odontodes, also my existing P. maccus, that didn't seem to have any before.
Am I going to be in trouble with three males?
At the moment the fish, that was already in the aquarium, is bulling the other two around quite a lot, even though I took it out and completely rearranged the furniture. Is this something I should be worried about, or do they just have to establish who's the boss and the 'fighting' will be over soon?

I would agree with you, that they all appear to have odontodes on the body, so you probably have 3 males. I don't think that anyone ever wants to have all of one sex together, in particular all males. So to be honest, I don't know if the aggression will go down or not.

I would think that if the tank is large enough that the males can keep their distance, and if you have enough hiding places and caves, then they should settle into some kind of pecking order. But I suspect you'd agree that the long term solution is (1) get some girls in there, and if possible at the same time (2) get rid a boy. Keep in mind that I have 3 boys and 4 girls in a 15 gal aquarium, and the males have almost no visible conflict with each other because they've all staked out caves. The only injured fish I ever see are the females, usually after a night of mating - recently, one female had seriously torn fins and small bloody wounds on her body sides after she spawned; these seem to be healing up though, so for now, I'm not too worried about her.

Cheers, Eric

For now, things seem to be alright between them. When they accidently run into each other, there's some dust flying around and they quickly move away in opposite directions, but they are not deliberately chasing each other. On the other hand, they are still settling in, so I'm not sure if the situation will stay this way.
All three of them are pretty active and they are not afraid to come up to the front of the aquarium (even when I'm sitting next to it), so I think that's also a good sign for now.

Deciding which males to keep will be hard (I really like the markings on the two new arrivals, so for breeding purposes it might be a good idea to keep them, but the other one is my first P. maccus, so it would be nice to keep him as well), so I'm considering to eventually move one of them to my other aquarium. This is 80x36x50 cm or 31,5x14,2x19,7 Inch (with the cover) with one female Ancistrus sp.4 and two female Ancistrus claro, which are all very friendly. So maybe there's some free bottom space for one P. maccus. If in the future, I'll have the room to buy a bigger aquarium for these guys, I can put them together again.

But I still have an even bigger problem; how am I going to ensure that the next P. maccus I buy are going to be females? I think that's going to be a very big challenge, especially since I don't have room for any more males.