Change toolbar 3 dot icon and how to get secondary toolbar items under a primary toolbar item.

Hi,
I have a toolbar that contains 4 secondary toolbar item. There is a 3 dot icon (which is by default), by clicking on which the toolbar dropdown opens. I want to replace the 3 dot icon with an icon of my choice. How can I do it?? Also if anyone can tell how I can add a secondary toolbar items under any primary toolbar item.