The idea of a 'new' house won't be a concept that your 3yo will be able to grasp in the way that you do. In reality, all your DS needs is you, and to be sure that you aren't going anywhere!
My DH and I were discussing 'moving house' recently and it transpired that my 4 yo (who is generally thought to be quite bright!) has always thought that when people move house they take the house with them! My 6yo, on the other hand (who has autism, therefore an excuse for this!), thought you had to leave everything behind (furniture, toys, etc), and was terrified that he'd lose everything if we moved! This just goes to show how much we can underestimate our childrens' abilities to misunderstand the language we use, and how hard it can be to anticipate what their worries are!
If you focus too much on the positives (being at the seaside, lots to do, etc), then you could be setting yourself up for alot of hassle when you first move if you don't find you have the time to do all those things and your DS is expecting an endless holiday situation!
Conversly, if you focus too much on 'addressing his worries', you could be putting ideas into his head and end up making him worry about things he hasn't even considered yet!
A child of 3 lives very much in the moment, and as the previous poster says, so long as he has his bedroom set up asap on the day of the move, plenty of toys to play with, plenty of good, filling food to fill him up and plenty of access to you for cuddles, security, etc, then he'll adapt just fine.
If I were you, I'd get on with the business of finding somewhere to live and not worry about involving your Ds until the move was imminent.