Two circles with centers at $\mathrm{A}$ and $\mathrm{B}$ touch each other externally at $\mathrm{T}$. Let $\mathrm{BD}$ is the tangent at $\mathrm{D}$ and $\mathrm{T}\mathrm{C}$ is a common tangent. If $\mathrm{A}\mathrm{T}$ has length $3$ units and $\mathrm{B}\mathrm{T}$ has length $2$ units, then the length (in units) of $\mathrm{C}\mathrm{B}$ is