I'm not a tarot reader, just a beginner at angel cards (using the past-present-future format) and an artist toying with the idea of designing/painting a deck. Since the idea of threes seems important to your deck, would a triangle layout structure work? Top of triangle could be the over-arcing theme, while the base of the triangle (whether it's 2 cards, 3, or more as you add more layers) would provide more details. I'm just throwing this out as a mental image that hit me, but not sure how it works with your 3 card groups.

Artwork, always yes. I've seen how readers can use it intuitively. Some spreads wind up having a dominant colour or two that can provide very on-point clues. I'm visual and find that if something specifically is drawing my attention, it's relevant to the reading.

I hope this is helpful instead of confusing!