SANAA, Yemen — Yemeni forces killed three men Saturday who were among nearly 60 suspected al-Qaida militants who escaped from prison this past week, officials said. The three escapees were awaiting execution in the Mukalla prison in southern Yemen, where inmates attacked guards, seized their weapons and fled through a tunnel on Wednesday in another sign that Islamic militants are making gains amid Yemen’s political turmoil. Nearly four months of anti-government protests have left the country’s president of more than three decades clinging to power.