Given \(\displaystyle\mu={M}{e}{a}{n}={30}\) minutes

Formula probability exponential distribution: \(\displaystyle{P}{\left({x}\le{a}\right)}={1}-\frac{{e}^{ -{{a}}}}{\mu}\)

Let us evaluate the formula at x=180 (as 3 hours are 180 minutes): \(\displaystyle{P}{\left({x}\le{180}\right)}={1}-{e}^{ -{{\left(\frac{180}{{30}}\right)}}}={1}-{e}^{ -{{6}}}={1}-{\left(\frac{1}{{c}^{6}}\right)}~{0.9975}={99.75}\%\)