Mathematical Bold Digit Three

It still means three; the formula supplies any extra role.

Mathematical notation uses bold and other type styles to distinguish symbols, so their role comes from the equation. Bold letters can mark vectors in physics or engineering, but that convention does not make 𝟑 a vector or give it one fixed meaning.

Some mathematical letter forms already had their own characters. A 1999 proposal expanded those scattered forms into consistent style families, digits included, so formulas could preserve the distinctions in text. The aim was mathematical notation, not general bold formatting. In LaTeX, \symbf{3} from the unicode-math package gives the bold math form.

Why does 𝟑 turn into 3?

Compatibility normalization can fold it to an ordinary 3.

Unicode text-cleanup modes called NFKC and NFKD map 𝟑 to the plain digit 3, dropping the bold distinction.