A particle of mass $m$, charge $Q$ and kinetic energy $T$ enters a transverse uniform magnetic field of induction $\vec{B}$. After $3$ seconds the kinetic energy of the particle will be:

The force exerted by a magnetic field on a moving charge is $F=Q(\vec{v}\times \vec{B})$. This force is always perpendicular to the velocity of the particle.Since $\vec{F}\perp \vec{v}$, the work done by the magnetic field is:
$$W=\int \vec{F}\cdot d\vec{s}=0$$
According to the work-energy theorem, the change in kinetic energy is equal to the work done. Since work is zero, the kinetic energy $T$ remains constant.