There is a concept known as "training stress". When you put stress on your system and then give your body time to recover, you will improve.
When you started, it was easy for you to put stress on your system, but over time, your body adapted, and now you only put enough stress on your system to stay where you are.
The solution is to change up what you are doing, to once again provide training stress. To do this, you will need to increase the intensity of the work that you are doing. You can't do this with your current workout, so you will need to change your workout to be harder (during the high-intensity parts) and then easier during the rest of the workout. I'd also start by reducing the number of workouts back to twice a week.
You can find lots of good running interval workouts online. I would also suggest that you consider fartlek workouts; they will likely to be more interesting.
Do the interval workouts for a couple of weeks, do one easy workout to make sure you're well-rested, and then retest the runs you are doing now. You will see an improvement.