No doubt I am stating the obvious to many of you – but for the benefit of the new user…

The 4-thread overlocker/serger is the most popular but this rule applies to the 3-thread also.
The most important thing to remember is the threading sequence.
1st. Top Looper.
2nd Bottom Looper.
3rd Right Needle.
4th Left Needle.
The needle sequence isn’t as important but you should always thread the top looper first and then when you thread the bottom looper, lay the thread over the threaded top looper.
I’ve tried to show this in the photo using a Bernina 800DL in our showroom.