Filing end gaps isn't hard to do. It can take some time.
As 650 skull says, a flat file works. Clamp the file in a vise. Set the open part of the ring over the file. Stroke the ring down the file. Put it in the cylinder. check gap. If too tight. A few more strokes on the file, re try.
To check the gap put the ring in the top of the cylinder, use a piston to push it down in the cylinder a bit farther till it's straight in the cylinder. You use a feeler gauge to measure the gap bwetween the ends of the rings.
Your repair manual gives the procedure and specs. End gaps are top .008-.016, second same as the top, the third .012-.035