In my opinion you should stick with the dogs you already have and not add another dog at this time.  The eight year old dog might find the adjustment hard to take.  If you have all of the dogs neutered it might be all right to get a male pup, but there is still the territorial problems.  Also, the pup may be aggressive and as he grows up, he could try to dominate the other two, causing big problems; and for the older dog, is that a fair situation to put him in?  A female added to this mix, does not seem to be a good idea.

i have always found females much easier to train.and i beleive females are more protective. possibly because they have that motherly instinct. but im sure there are alot of really great male dogs.good luck.but dont buy your pup from anyone on line that wants you to send money western union.there are too many scammers from africa ripping people off.i know they tried it on me.so i just kept stringing them along with many exscuses...i think i tired them out...hee hee.