He needs to be properly introduced to your cats, and this will take time, and a lot of training, and consistency.
1. First, create some training treats that are highly reinforcing to the new dog. Little bits of chopped cold cut meats, and cheese work well.
2. Before he is allowed around the cats, he needs to be taught some commands, FIRST. Here is where the training treats come in handy.
-Work on "sit", "stay", and "come", "quiet" first. Use the command only once, and expect compliance before offering a treat. Lots of web sites will help you with specific techniques.
-If he barks at the cats, use the "quiet" command he has mastered, and then offer a treat upon immediate compliance.
-If he likes to chase your cats, you may have to work on bringing him into the area of your home where your cats are at on a leash, tether him to a door, and work on the "sit" or "stay" command, offering a treat for immediate compliance.
-After a he seems to master these commands, you are ready for some supervised visits without the leash with your cats. If he chases, barks, or does something else unacceptable with the kitties, he needs to be removed, or tethered again, once again working on his commands with reinforcement.
*Be patient! He's a 3 year-old dog, and while still trainable, will need a lot of time to get over his curiosity of the cats, and a long adjustment period.
*Do not yell or scold your dog for being "rude" to the cats...all you will accomplish is a dog that's afraid of you. Positive reinforcement and assertiveness works much better than anger and fear tactics.
*While tethered, allow the kitties to sniff him, hiss at him, etc...but no scratching or batting allowed!
Good luck...it's a long process, but one that is necessary. Again, do some searching on dog training sites for some tips and tricks.