The honest is, I don't know. But I shall do my best to find out.
Typically Hub 3 should have only the base LED lit when it's functioning normally. Although the other lights tend to be active during the boot up sequence they should be unlit after a couple of minutes, and remain unlit.
Which of the 3 green lights is flashing - wireless, Internet or phone?
Please restore the Hub back to factory settings and let us know if that makes any difference in the light behaviour.
Also, aside from the annoyance, is there any impact on broadband performance?
I'm sure I've dealt with another issue like this recently. My advice will be the same - if your Internet is working well then try to ignore the LED (or put tape over it). Meanwhile I shall see if I can locate the other issue and cross-reference for any points of similarity (aside from the Hub type and flashing LED that is).
Of course, if you experience any broadband related problems just post an update on here and we'll investigate for you.