
I am presuming that your pup is now much bigger than the 3 smaller breeds mentioned so they may be very wary of him because of that, I would have opted to introduce him to them first as a young pup rather than Max who had problems, you are going to have to take it slowly.
Yes neutral ground is best,one at a time starting with the female, 2 people taking them out for a walk will allow them to be together but in a controlled way and see how that goes and take it from there it may take a while to get each dog to be comfortable with him but never leave them unsupervised with him as they may gang up on him.
Hopefully others will be along who have actually been in this situation as I have only introduced dogs in the same breed to each other bar for introducing Rhuari my 9 week old dachsie to my 15.5yr old cocker and then the adult was much bigger than the pup.
Keep us posted on how things go