I don't even care if you don't believe me or not, don't have anything to proove to you anyway, and yes I still claim that i've been using 3dsmax in 7 years and 3 years of maya in high school.

Not 'knocking' 'your' English.
Just teasing back.

On how to, haven't a clue, however, here's what a dummy might suggest:
1. export to Blender (.3ds or .obj?)
---download
---goto wiki
---read a bit
---get shortcuts
2. mark seams
3. click monkey face button
---unwraps it all neatly, almost automatically
4. export to Deep Paint 3D (.3ds or .obj?)
---as mentioned by Landixus
---leave Blender open
5. load up textures
6. paint in layers
7. collapse layers and export to 3DGS-supported image
8. in Blender (again), export to mdl using Ventilator's mdl7 plugin (.mdl?)
9. load image into MED
---use version of MED which auto-resizes UVs to image on image import


Quoting Enduriel.

'I' don't care if 'I' believe and / or don't believe 'you' either.

Hope that was helpful, but it probably wasn't.

I'll try that out. Thanks for your efforts >_^

the link for the 'monkey-face' button:
LSCM unwrap

Probably, 3dsmax already has something like that.

yeah I used pretty much the methods you said and achieved this but in 3dsmax, so texturing model based levels does work but with some kind of gimmicks.

Had to first paint red,green,blue,black as vertex paint on different channels, then add the corresponding colors as materials to the same channels. Then unwrapp the thing nicely (hate to uwnrapp terrains V_V), and then render to texture diffuse, then the shadowmap, and this is what I got in 3dgs with the shader from Slin & co from the free outdoor weather dynamics+shaders pack.

Pretty nice if u spend some time on the terrain if you ask me since I did it in 5 minutes just to try to actually get it to work in 3dgs a7.