
Hi Kate,
as Maxine has already pointed out you need to look at the practicalities of walking three together and transporting that number together too. Obviously the size of breed has a more significant impact here unless you're talking about a toy or small terrier.
The costs go up too, not just food but vets, kennels and insurance if you use either of the latter. Getting family, friends or a dogsitter / house sitter in while your unavailable (think emergency!) can be more tricky too.
The age of your current youngest
could make it slightly more difficult to train and bond with a puppy - being so young himself, your current male will probably quite quickly become best friends with the new arrival (for a while at any rate!) and although it's great fun to watch them play and rough house it's sometimes difficult to get the opportunity to keep up to speed with the training *separately* of both :)
It's difficult to predict how well they will all get on as your current males has not yet fully reached mental or physical maturity so his temperament is not yet set in stone and of course your new addition is an unknown quantity.
It's a judgement call TBH but I would personally recommend a larger age gap before adding a new pup to the mix. I've generally tried to keep in the region of 4-5 years between my gang (generally 3 but at one time 4 mixed s#x) and still always found the youngest member delighted to play and buddy up with a new arrival :)
Give it some serious thought and try not to let your heart rule your head. HTH, Teri