It would often be quite impossible to determine which team scored a goal, for instance in 'scrimmage' situations, or own goals.
Plus it would create weird situations where a attacking team would stop a goal from the other attacking team on the line and try to get in on their own name.
That's a problem in 2-team football, anyway (the individual who scored always has to be identified), although the point you raise about own goals is an interesting one. Maybe an own-goal should award half a goal to the other two teams?
Plus it would create weird situations where a attacking team would stop a goal from the other attacking team on the line and try to get in on their own name.