As Avon says, you can only apply textures on a per surface basis, not per triangle AFAIK.
So, the only ways I can think of to achieve what you want is to either sort all triangles into separate surfaces, or combine your textures into one large texture and UV map each tri appropriately. However, even then you'll probably have to unweld a load of tris to get the effect you want.
That's a lot of fiddling and hassle so probably isn't a practical solution. Best I can think of, though.
So, the only ways I can think of to achieve what you want is to either sort all triangles into separate surfaces, or combine your textures into one large texture and UV map each tri appropriately. However, even then you'll probably have to unweld a load of tris to get the effect you want.
That's a lot of fiddling and hassle so probably isn't a practical solution. Best I can think of, though.