what is the number "3" on Mantra of Destruction skill icon?

Sometime I don't know how, but as the combo reached, when I use Mantra skill, I could see a number 3 with a circle appeared on the Mantra skill icon, and my other skill could use that number as an Empowered attack count?

But I don't know why it disappeared .

Please tell me what mechanism behind this.

Do you or did you use "Empowered Sparks" support gems? Cause it do what you explained.
Otherwise i guess it's another support that do the same and that you maybe removed.