What is Salamanca reaction to the loss of his dog ?

There is no character by this name in "The Stranger."

Why are meursault , Mari , and Raymond spending the day together at the beach house of a friend of Raymond's ? 

Salamano lost his dog at the parade grounds. Raymond tries to reassure him that if the dog is lost he will come back. Salamano is worried the police will get him. Meursault explains how he can pay to get the dog out of the pound which Salamano finds absurd (paying for a dog he does not even like) but questions Meursault about it later. Meursault explains how the pound keeps the dog for a short time before deciding what to do with it. Salamano leaves but Meursault can hear him pacing and then crying.