The three brightest stars in the Leo constellation are:

1. Regulus (α Leonis): This is the brightest star in Leo, with a magnitude of 1.35. It marks the heart of the lion.

2. Denebola (β Leonis): The second brightest star in Leo, with a magnitude of 2.14. It marks the lion's tail.

3. Algieba (γ Leonis): This is a double star, meaning it appears as two close stars. It's the third brightest star in Leo, with a magnitude of 2.01.