Hey Herve!
You have a really nice piece of music here, and before going into the technicals, having an idea of where you want to go could greatly help.
Three-part epic instrumental, cinematic, or any other term is very generic and won't, in my mind, get you the results you want to achieve.
Let me give you a quick example: The soundtracks of Lord of the Rings, Pirates of the Caribbean, and Frozen are all cinematic music. Do they "feel" the same?
So in order to better get to where you want, I suppose you could start by picking the pieces you like and trying to emulate the feeling.
Having said that, and sorry for the long introduction, here are a couple of things that could help you achieve more using orchestral samples:
1. Don't play the orchestra as a piano player would; instead, split it into its core instruments and play each section on its own, especially the string section.
2. Use the different articulations, just like Telefunkin suggested, to highlight different parts. Using one articulation all through is like playing the same note over and over.
3. The orchestra has been designed to cover the frequency range, from the tubas in the lows to the piccolo in the highs (other than the percussion section). Use it to your advantage.
4. Orchestration is another subject that could elevate your tunes as well. Some instruments work well together; use them.
I suppose those ideas could be enough to get you started.
If you have any questions or if any of that needs more clarification, feel free to ask
