You can use this free program: http://www.gimp.org/
The 3 part probably won't work becouse you need to separate the pices and it would look like someone choped the horse. This kind of efect works if you can have the space between the images and think of the image as if you are seeing it in 3 different windows.
If you want to go:
1) Make a backup of the photo
2) Use the menu Image > Canvas Size > and play with the values
Regarding the size you need like 150ppi for a decent print. 200 ppi is the ideal for a photo.
If your file is 2000px width you can enlarge it lets say (2000/150=) 13.3 inches.
You can risk a bit resampling the photo you will have a bigger photo but a little blurry.
1) Image > Scale > 200%
2) Apply sharpen: Filters > Enhance > Unsharp Mask Play with the values.
And decide the photo size without further rescaling. Just play with the Print Size.