i have two dalmatians, they join me on my 3 hour hack everyday, with my horse.
I recently brought a third, when i introduced him to my horse, he freaked out, he wont go near the horse atall now!
This is a big problem as this is how i exercise my dals, and they love it, how can i make him see the horse is a nice thing!??
(he is 4 months old)

You will have to desensitise your dog to the horse until it changes his reaction from "OMG" to "yippee" when he sees one.

Basically gauge at what distance yoru dog feels comfortable to the horse eg 100 ft away and click and treat his calmness with a treat that is of incredibly high value and ONLY available in the presence of the horse.  Over a period of time, do not rush it, you will be able to reduce the distance between them until the sight of the horse itself becomes the trigger for him to say to himself "oh goodie a horse great, I get a chicken wing, liver cake etc etc"

It does take time but if you are careful it will be no problem. Make sure that his reward comes at NO OTHER TIME.

Don't force the issue! You have plenty of time to get him used to the horse because he won't be going for three-hour hacks till he's an adult. Just have him around when you're grooming, mucking out etc. You could try putting his dinner down in the stableyard while you hold the horse at the other side. Walk him on his lead at a distance from the horse that doesn't bother him. As the days pass this distance should become less and less. What is vital is that you don't force him closer than he's happy with.

Hope this helps.