I have 2 border collies. One male, one female.
The male one loves to romp around and loves rough play with other dogs. The female one is quiet and calm and loves to play with same sized dogs but doesn't enjoy rough play.
So when I go to the dog park there are certain large dogs that have figured out that my female dog gets intimidated by barking. So the 3 individuals in question, if any one of them is present, will start by running after her and barking until she flees into a corner and sits down.
Then they keep barking and cornering her. It's pretty obvious that she does not enjoy it. I consider this bullying. There is no play going on anymore once she sits in the corner.
Usually I leave them to it but today a particularly unpleasant individual briefly grabbed her and she yelped so I went after them and in between until finally one of the other dog owners told the owner of the bully to call his dog off.
So what's the best way of dealing with this kind of thing at the dog park?