If&nbsp;<span class="math-tex">\(\vec{a}, \vec{b}\)</span>&nbsp;and&nbsp;<span class="math-tex">\(\vec{c}\)</span>&nbsp;are unit vectors, then&nbsp;<span class="math-tex">\(|{a - b}|^2 + |{b - c}|^2 + |{c - a}|^2\)</span>&nbsp;does not exceed