I don’t undestand why dist©=3 because the kernel can have many vectors.

ty

The kernel does in fact have a number of vectors. However, if you find the distances between all pairs of vectors in the kernel, the minimum distance will be 3.

\mathrm{dist}(C) \triangleq \min\left\{ \mathrm{dist}(x,y) \Big| x \neq y \, , \, x,y \in C \right\}