A deck of 52 cards doesn't divide evenly into threes. How do you handle the leftover?

52 ÷ 3 = 17 triples with 1 card left over — encode the last card as a simple standalone image (or switch that group to a pair).

Perfect division isn't required. Options for the remainder:

• Make 17 triples (51 cards) and store the 52nd as a plain single image at the next locus.
• Or mix in a pair so everything is covered (e.g. 16 triples + 2 pairs).

Either way you just need a consistent rule for the tail so decoding stays unambiguous. (Pair-based systems avoid this: 52 is exactly 26 pairs.)

Go deeper:

• Dominic system (Wikipedia) — the pair alternative (52 = 26 pairs) sidesteps the remainder.