A particle moving with kinetic energy $3J$ makes an elastic collision (head on )with a stationary particle which has twice its mass, During impact:

 

In a head on elastic collision between two particles , the kinetic energy becomes minimum and potential energy becomes maximum at the instant when they move with a common velocity. The momentum and energy are conserved at energy instant. $\frac{1}{2}m{u}^2 = 3Jmu = (m +2m) v =v =\frac{u}{3}$

Minimum kinetic energy of the system $\frac{1}{2}\left(3m\right){\left(\frac{u}{3}\right)}^2 = 1J$

Maximum elastic  potential energy of system = (3-1)J = $2J$

During the course of collision ,due to deformation elastic potential energy of the system first increases and kinetic energy decreases , after maximum deformation the objects gradually regain their shapes and as a result elastic potential energy decreases and kinetic energy increases.