One way is to use the GrowFX for placement of trees on the surface, as instanced objects.
So, you can make 3 or more different trees. Then, create GrowFX object and distribute the path from some surface (see picture below).
Add the "Instanced geometry" mesh-builder, and add these 3 trees. Turn off the "Combine meshes", and turn on the "Display as bounding box", as a result - the models will be distributed as instanced objects.
Download this scene: Multiple_trees_02.zip
Also you can use other plugins to distribution of multiple trees on the surface:
"Forest Pack Pro" from www.itoosoft.com
"MultiScatter" from www.rendering.ru
Good luck