What should my brother do about the 2900 riyals compensation due to the other party in the car accident, which has not been collected for three years, especially since it is impossible to obtain information about the other party from the traffic authorities?

If you despair of reaching the rightful owner after exerting your utmost effort, then give the money in charity on their behalf. If the rightful owner appears thereafter, they have the choice between ratifying the charity and receiving its reward, or taking their right, in which case the reward of the charity will be yours.