2002 Honda Civic Slipping out of third gear

I recently purchased a 2002 Honda Civic 1.7L 5spd standard and when I shift to third gear and let out the clutch and push the gas the car pops out of third gear followed with a grinding noise, also there is a loud noise when the car is in reverse while reversing, I know it is usual for the car to have a little whining noise while reversing but it sounds rougher and louder than normal and it is hard to shift in reverse, what is the problem is it in the gearbox or transmission, what will need to be done to fix this problem?