$\"\"$

The statement is false. All three vectors may originate at the same point and not form a triangle at all. If so, the angle between $\"\"$ and $\"\"$ may be acute, right, or obtuse.

$\"\"$

$\"\"$ , $\"\"$ , and $\"\"$ form a Pythagorean triple, and the angles between $\"\"$ and $\"\"$ and between $\"\"$ and $\"\"$ are acute, but the angle between $\"\"$ and $\"\"$ is obtuse.

$\"\"$

The statement is false. All three vectors may originate at the same point and not form a triangle at all. If so, the angle between $\"\"$ and $\"\"$ may be acute, right, or obtuse.