Numerology's number-to-trait assignments generally trace back to older systems of symbolic meaning — Pythagorean number theory, astrological correspondences, and centuries of practitioner writing that built on each other. 3 tends to inherit associations with communication and creative output across most of that lineage, which is part of why the "expressive" reading is so widespread rather than being any single author's invention.
In practice, modern numerology books describe 3s as drawn to writing, speaking, performing, or any outlet where an internal idea becomes something other people can see or hear. Some sources add a caution alongside that: the same restlessness that fuels expression can also read as scattered focus if it isn't given somewhere to go.
None of this is measured. It's a reading — a consistent one, passed down and elaborated on, but a reading nonetheless.