Yes, if the angle between the two vectors is more than 90^o but less than 270⁰. (cosΘ is negative)

The two vectors (say A and B) of different magnitudes cannot be combined to give zero resultant since minimum value of combination is ІA-BІ which is not zero if AB.

The three vectors A, B and C of different magnitudes can be zero such that they form a closed triangle, then,

      A+B+C=0

or, C=-(A+B)

Hence, the sum of three vectors may be zero if vector sum of any two vectors is equal and opposite to the third vector.

Note: The vectors can give this result only if...

Balance on a bicycle is a matter of constantly correcting against falls, and it's easier when the speed is higher because the inertia of moving forward overcomes the need for corrective actions.