As Whickerman says, originally getting over the goal line did not in itself result in any points but gave the team concerned a "try" at goal, that is kick the ball over and between as now. After a time it was thoughs that the efffort in actually attaining a try was worth some points so it was given as 3, with 1 point if the try was subsequently converted to a "Goal". fast forward to today we have a goal now being worth 7 (5+2), and a penalty goal or drop goal worth 3.