This to go in a tab. The other tabs show quite separate groups the user might find interesting. This tabs contains all of them.
I've thought about writing "ALL" (capitals seemed more recognisable in this case), but there are icons for all other things, and it makes it harder to translate the application.
Another thought was three checked checkboxes on top of each other, but that seems more like a form to fill in.
If it is for tabs, I would use icon symbolizing cluster of all things, either something more relaxed or less relaxed , depending on overall app style.
Also the target audience is important for the decision, because some audiences may already have their 'all' symbol. For example for advanced computer users, 'all' can be expresed by asterisk (wildcard *).