OCD is extremely common in warmbloods, over 30% will have evidence of an OCD lesion somewhere. If a breeder tells you they have never had an OCD in any of their offspring they are either lying or they never Xray. We do screening X-rays in the fall/winter of the yearling year. Some lesions will go away, some we operate on. I have never, ever had a problem down the road on a horse related to an OCD lesion that was properly treated. OCDs are best operated on at a fairly young age so that the area of the lesion heals as the bone continues to grow.
In your situation, you have a slightly older (3) year old horse that is already lame due to the lesions. I would recommend that you consult a very knowledgeable vet who is very used to dealing with OCDs. The prognosis depends on the size and location of the lesions, and your accessibility to a vet very accustomed to operating on OCDs. The prognosis for your particular horse is not something you can get off the internet, unfortunately.