I would not recommend a 300 for softball and certainly not a fixed 300. If I had to pick one lens for softball, it would be a 70-200 f2.8. The real answer depends on how serious you are about shooting, how old the kids are and what kind of access you will have. One reason I ask about the kids age is the size of the people you will be trying to frame. Another thing that also ties into that would be potential for long term shooting. Do you expect them to play for a few years or 8 years? Do you hope to continue shooting them as long as they play? Do you expect to be shooting from inside the fence, outside the fence, in the dugout? All of those answers will play a big part in what I would recommend...The simple recommendation is to buy the best glass you can afford.
If they are younger, you will have the advantage of mostly day games until they get older. By the time they are in high school, you will be shooting quite a bit under the lights. You will definitely want a 2.8 aperture at that point. Yes, I shoot baseball and other sports under the lights with my 200-400 f4, but it is really pushing the limit of low light capabilities on a camera that has better low light capabilities than the D700.
As far as a 300 prime lens, you might be able to pick and choose some shots and get some great results and nice close ups, but there will likely be as many times when you will want wider than the 70mm compared to the times you will want longer than a 200mm. I can crop out dead space on an image taken with a 200 but I can't widen the coverage of a 200. You will also find some value in having a zoom to allow you to follow the action better.
If I am limited to choosing from the 3 lenses you listed, the Sigma would probably be the best choice. Superzooms (28-300) generally do not provide the quality of shorter range zooms and the aperture will be much worse to deal with than the f4. However, as stated, you will probably find more times you want wider than 100 than times you will use the 400.