When dog owners decide to add more dogs to their home pack, they often blindly assume that if two dogs co-exist companionably, a third or fourth will also adapt well, and they will form a homogenous group. This kind of thinking only leads to disastrous reality. Pick up all toys, chew bones and bedding from the area where the dogs will meet for the first time. Remember, crates are your dog's dens, their place to get away from it all, and they can be very protective of them. Have all dogs on lead, new and old.
Some people prefer to introduce new dogs on neutral territory.
They haven't been looking for a roommate. Your old dogs are only exhibiting instinctive caution when they look suspiciously at a new dog. This is normal canine behavior. The owner must handle this tense situation correctly in order to see that the new dog fits in safely and happily. Instinctive canine behavior has been masked and modified by the domestic dogs long association and bonding with humans, but it's still present within the dog.