Adopted Older JRT with two other terriers in house

Hello-

My husband and I recently took in a 12 yr old JRT from a person who was moving and couldn't take him. We already have a male wire fox terrier(3 yr) and a female JRT (4yr). My wire fox terrier is very submissive and the JRT is socialized well. The new JRT gets along fine with my two dogs except for when he is in one of our laps. Then he shows agression towards the other dogs if they come near. He is also marking which is another issue. I think that these problems are the result of his original owner not spending enough time with him. Should we take him to obedience class? What suggestions do you have for this situation.