You can definitely benefit from 2 shorter long runs back to back on weekends. We do this ourselves often with coaching clients especially we approach a big goal. The first day’s run leaves you a bit glycogen depleted so you start the next days long run pre-fatigued which will increase the aerobic training benefit.
As for not running longer than 3 hours. One of the biggest aerobic training stimuli is intra muscular glycogen depletion. If you are well fat adapted and staying Z1-2 you may not experience significant glycogen depletion in 3 hours. So, this can be a very individual thing. If a 3 hour run leaves you feeling pretty drained and you are feeling your legs and general fatigue 48 hours later then it clearly had a major training effect and did its job. If you can do back to back 3 hour runs for a week without feel wrecked then you’ll need to make your long runs much longer.