Let us say I have a square plate within an assembly, this plate is saved as a part. Using the Split feature, I create 3 plates out of this square plate, thus getting 3 parts. Now, I can open each plate individually, the files are saved externally in directory. So far so good.
But in feature manager of the original assembly, there is still only 1 part-->the original square plate that contains a sketch and the Split feature.So now I have to supress the original square plate, insert all 3 plates within the assembly and mate the plates in order to get 3 individual parts as I would like. Is there any shortcut to this? Why doesn't the program automatically list 3 parts after using Split, so one could quickly manipulate with them.
It sounds like you've managed to find a moderately suitable solution to your problem already, but that you don't like that it's messy and annoying. One thing you might try is creating different configurations of the original part for using in your assembly. That way, you could have one configuration with the whole square plate (if you still need it), and then one for each of the 3 individual parts.
With regard to "split" still resulting in only one part, it sounds like you're wanting the part file to basically say "okay, now I'm an assembly instead." Rather than creating three individual parts, the operation has created three individual, separate bodies within the one part. You're still able to work with these on their own, as it looks like you've done already.
If you really want these three parts that make up the plate to serve as an assembly file, why not create one to use as a subassembly in the main one? If there are just three, they'll be really simple to constrain, especially since they'll all have the same original datum planes.