Considering you have materials in a location on your device or you have used the texture resources quick links and have downloaded these materials to a location> On the I-Mixer panel in Group A press the "load image for........" button> navigate to material location> Select desired image textures> Then press "load images" button> Then press "Create Materials for Group A, B or C" button pertaining to the group you are working with to apply material to object.

TIP: (Subdivide: to increase vertex count in order to paint material/details in vertex mode)

When images are loaded and applied for the group you are working with there is a toggle check box below the "Create Materials for Group A, B or C" buttons. When triplanar mapping is enabled, I suggest adjusting the mapping scale value to desired size, & adjusting the blend value for seam blending control.

In the sidebar pertaining to the materials group the "Height Blend Settings" value and "Color Ramp Controls" Settings that will help you achieve hard edges on a blended material, or soft faded edges for a more organic/realistic look.