Hi Loopeozzie, I think for a forrest, my opinion to to visualise how a small forrest would look. Larger trees in the middle representing older, more mature trees of the forrest, growing taller searching for light, shorter or smaller trees around the outside representing younger trees in the forrest. This is my image only, others may have a different idea. Then the idea is to have an odd number of trees, one of the Japanese rules for asthetics. Then arrange them in such a way as to look "natural", and preferably so that no tree blocks the view of another tree. Also below it appears that all are mounded quite high, not sure if this will be sustainable for such young trees as the soil will wash away easily when watered with very little root development around their base.
Elias, what little my opinion is worth, I think the young age fo trees is not overly important, i think it just depends on the sixe of the plantation you are after. I think youch trees can be suitable, arranged well and trimmed well...
Oh, another thing, remember that with trees in a forrest, the inner trees will be more sparse of foliage, less branches, the tree is spending more effort growing high to reach light, the outer trees will have more branches (growing outwards) and lower down branches.
For what you have here, I would say add at least one more tree to have 5 as a minimum. Otherwise 3 in a forrest might be too small, with one travelling solo.
That being said, When I drive to and from Geelong I notice groups of 2 or 3 or 4 peppercorns growing on the side of the roads with one or two dominant trees, and the others growing at radical angles and lines, absolutely complimenting the scene...
Feed, water etc as with your plants normal requirements... keep us posted how you go.