Our last five week trip was based around being in a particular location so we could pick up a nine week old Collie puppy. We made sure most of the trip was concluded, having gone from SW Florida to Las Vegas, Arizona, Hot Springs, AR and St. Louis. We had only two more stops before heading home when we got the baby.
We had little to no problems with the pup. Before leaving on the trip, we purchased a used car carrier large enough to hold a beagle and kept the pup in it when sleeping or on excursions where the dogs could not go with us. We also had a 13 yr. old Sheltie with us. The biggest problem we had was getting the pup to sleep the first couple of nights. We made sure the pup had been wormed and given his first set of puppy shots by the breeder before we picked him up. We kept carpet mats down outside the coach where the puppy could play and do his business. The trip home was actually enjoyable with him.
I agree that no puppy should leave his mother until the age of at least seven or eight weeks. We got our Sheltie when he was less than five weeks old from a backyard breeder who wanted to dump the pups as fast as she could. This caused emotional stress with the pup, who bonded only with our female Aussie and made her his surrogate mother. It took him two years to bond with us. Eight weeks is a good age to separate the pup from the litter, especially if the breeder has been conscientious enough to socialize the pups before they are taken to their new homes. Be sure he has his puppy shots and has been wormed, and crate him when he is not being given the run of the coach. If you have chosen a three month old, you're probably doing the breeder a favor, since they are more difficult to sell than the babies. We got our last Collie when he was four months old and he turned out to be the best dog I have ever owned.