MB3 v3.0.5874:
In the player controls (which is in the "top panel" in my setting), if I grab the volume slider marker and drag it to raise the volume, but then before I release the mouse button I continue to drag over to the Equalizer and DSP settings icon, and THEN release the mouse button, the Equalizer and DSP settings window will pop open. (The volume is correctly maximized however.)
And, once the Equalizer and DSP settings window is open, clicking the icon in the player controls again won't close the window like in MB 2.5. (Not sure if that was intentional behavior for MB3.)
Also, the word "Equalizer" is misspelled: on the window header is shows "Equaliser" - with an s instead of a z.