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 $\vec{F}$ acting on a charged particle moving in a magnetic field $\vec{B}$ is given by:
$$\vec{F}=Q(\vec{v}\times \vec{B})$$
Key properties of this force:The magnetic force is always perpendicular to the velocity of the particle ($\vec{F}\perp \vec{v}$).The power $P$ delivered by the magnetic field is:
$$P=\vec{F}\cdot \vec{v}=0$$
Since the power is zero, the work done by the magnetic field on the particle is zero.According to the Work-Energy Theorem, if no work is done on the particle, its kinetic energy remains constant.Therefore, the kinetic energy after $3$ seconds (or any duration) will remain $T$.