Calculating Average Velocity
Average velocity is calculated by finding the total displacement of the body and dividing it by the total time taken to cover that displacement. In this case, the body moves along the sides AB, BC, and CD of a square.

Calculating Total Displacement
The total displacement of the body can be calculated by finding the vector sum of the displacements along each side of the square. Since the body moves along the sides of the square, the total displacement is equal to the diagonal of the square.

Diagonal of the Square
The diagonal of a square can be calculated using the Pythagorean theorem. In this case, the sides of the square are given as 10 m each. Therefore, the diagonal can be calculated as follows:
Diagonal = √(side^2 + side^2) = √(10^2 + 10^2) = √200 = 10√2 m

Calculating Total Time
The total time taken by the body to cover the diagonal of the square can be calculated using the formula:
Time = Distance / Speed
In this case, the distance is the diagonal of the square (10√2 m) and the speed is 3 m/s.

Calculating Average Velocity
Average velocity is given by the formula:
Average velocity = Total Displacement / Total Time
Substitute the values of total displacement and total time into the formula to calculate the average velocity of the body moving along the sides of the square.
Therefore, the average velocity of the body moving along the sides AB, BC, and CD of the square with a constant magnitude of 3 m/s is 3√2 m/s.