It may not be obvious, since there are only 3 led's, but if you look more closely you should notice that the led's also reduce to half-brightness too. So, here are the 5 levels:
- All 3 led's full-bright
- Top led half-bright and other 2 full-bright
- Top led off and lower 2 full-bright
- Middle led half-bright and bottom led full-bright
- Only bottom led full-bright