I don't know your history with this machine so some of the following may seem basic beginner.
Be sure you have the right bobbin. This machine uses a larger bobbin than other Brothers. The manual also indicates some x3817 came with "quick set" bobbin feature for which lower threading is slightly different.
If you need a manual for reference, it's free download from Brother at;
Extremely high top thread tension also may cause upper thread looping. You have tension at 3 which should be fine but there may be other issues affecting top tension.
Test top thread tension with needle up, presser foot down and tension at 3. Make a long slow steady pull of the top thread. The thread should advance smoothly through the needle eye with out snagging. Any sudden change in tension is suspect.
You can do a similar test of bottom thread. Bring bottom thread to top. With needle up, foot up, make a long slow steady pull of the bottom thread. Bobbin thread should advance from the hook case/bobbin smoothly with tension similar to the previous top thread test.
In general, the top and bottom tension should be near the same when top thread tension is set at 4. If you find an extreme difference in top/bottom thread tension at 3 then something is amiss. Re-thread per machine instruction.
I suspect top thread has looped around something or there's debris in the bobbin area.
If still a problem comment here with your findings and I or someone will assist further.