Three cards: find the main thread
A familiar structure is past, present and future tendency. It works well for a focused current question: the state of a relationship, the trade-off inside a decision, or the direction of the next phase.
A complete three-card reading should explain every card in its position and connect all three into one answer—not present three unrelated dictionary meanings.
Seven cards: map the layers
When one question contains an inner need, outside influence, obstacle and hidden resource, a seven-card spread can give those layers separate positions. It suits a question you have already been carrying and want to explore more deeply.
The extra cards are not there to make the reading more mystical. They prevent several important clues from being squeezed into one symbol.
A simple way to choose
Choose three cards when you can state the question in one sentence and want the most important thread. Choose seven when you understand the surface situation but still want to know why it feels stuck, what resources are present and what tendency is emerging.
If a three-card reading reveals one unfinished layer, you can deepen the same question with seven cards. If a completely different question appears, give it a separate reading.