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.

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)

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.