
Having had a lot to do with Dobes (friend/neighbour has owned 7 with up to four at a time). Certainly never ahd an issue with owning 3 bitches at once with and without a male.
3 of these have been male, and one especially and absolute pussycat, he loved coming round to let litters of puppies 'play with him' he was just besotted by pups.
I have also looked after friends of friends dobe bitches in with mine no trouble.
The main issue with a male is other dogs, as your boy is bigger it will get the blame if any male aggro starts, he may also feel the need to 'protect' his girls from other males advances.
Also male dobes can be dog dominant. so if your used to relaxed interactions with other dogs on walks, owning a male of any breed will have you checking out other dogs much more, as some male dogs will not be tolerant of other male dogs, even if yours is a pussy cat.
I have never walked more than two dobes at once on lead as they are just too strong for me, even with head-collars. This might be a consideration if your getting a third, that you will need to walk in shifts, and who will end up staying back alone?
Owning bitches (medium size Spitz breed) only (with only males visiting/staying occasionally), I find walking the girls much less stressful as very few other dogs will bother your dogs (meaning aggression, not romance) other than in my experience Staffie or Boxer bitches, who don't seem averse to a bit of argy bargy, but fortunately mine take the moral high ground and ignore them (except for Inka, but she's a primadonna, and all mouth and no trousers).