An object started to slide down an inclined plane when angle of incline (also called angle of repose) is just equals to angle of friction. Let θ= angle at which a mass started to slide down an incline.
Then, force of friction = downward force. ⇒µN=mg‌sin‌θ ⇒µmg‌cos‌θ=mg‌sin‌θ ⇒tan‌θ=µ ⇒θ=tan−1µ Hence, angle does not depend on mass of body. So all three masses slides down at same instant at same inclination angle (θ).