My dog is aggressive to only 3 breeds of dogs. She got into a fight in a dog park the other night and I don't know what to do. Any suggestions?

I'm hoping you can help me, I have a 18month old Staffy x Bull Mastiff who I adopted from an animal hospital when she was 12months old. Initially showed signs of fear aggression towards other dogs, most all dogs. I figured this was due to her street life and shelter life. The shelter does not adopt dogs that are aggressive. Working with a trainer and continuing to we have been quite successful in removing this from her behavior however i have noticed that 3 breeds set her off. I have been trained to look for various signs etc and the trainers are quite good however I want a second opinion. She is aggressive towards German Shepherds, Huskys and Border Collies. We thought maybe shape or hair length but she socializes well with Goldens, staffys, kelpies, you name it but those 3 dogs set her off. I am confused as to why, I am taking steps to correct this behavior. She is fabulous with people and generally a big softie but around those breeds is a different dog. Strangely she will play with a GS x Kelpie all day and be submissive at times but never aggressive. I feel that something in her first 12months of life has obviously caused this. I write to you because the other night she was in a park when a GS entered the park, both dogs off lead, other dogs around and made a V-line for the GS.Ran at least 100m to get to the dog. Didn't attack initially, circled around then jumped. Naturally i ran over but not in time, they had been at it for about 3-5seconds before i puller her off the other dog. Both were fine this time (i also read your article on how to separate fighting dogs) and I guess I'm lucky to not have any marks on me. I am totally confused as to why this has occurred and what i can do. Any suggestions would be greatly appreciated. We have been attending classes for about 3months with great results and will continue to attend for a 12month period, longer if necessary. I am dedicated to the process and the animal. If you do get an opportunity to reply, can it be via email or please direct me to useful articles. Thank you.