The problem can be solved by using only the conservation laws and 4-invariants: if p_\nu and q_\nu denote two 4-momenta then p_tq_t-p_xq_x-p_yq_y-p_zq_z takes the same value in all frames of reference.
Please submit the solution to this problem via e-mail to physcs.cup@gmail.com. The next hint for the Problem 3 will be published at 13:00 GMT, 6th February 2022, together with the next updates of the intermediate results.