Without a shader, you are a bit stuck. Tri-planar works by using three planar projection for uvs ( one for each world planes : XY, YZ, and XZ ) and then blend the three layers with the normal values pow by some value as coefficient.
What are the options you have to render your terrain, are you allowed to edit the geometry ? Can you do multi pass rendering with alpha blend ?
Everything is shader, why are they inaccessible ?