Final answer:
Three U.S. senators who have been elected president are Barack Obama, John F. Kennedy, and Warren G. Harding.
Step-by-step explanation:
The question asks us to name the three sitting U.S. senators who have been elected president of the United States. Historically, most presidential candidates come from positions like governors or senators, with a significant focus on name recognition. Barack Obama, a junior senator from Illinois, successfully ran for president, winning the 2008 election. Another U.S. senator who became president was John F. Kennedy from Massachusetts. The third senator to achieve this feat was Warren G. Harding of Ohio. These individuals leveraged their legislative experience and national visibility to launch successful presidential campaigns.