Using the Sublime Text 3 menu, you can find EDIT -> Code Folding -> which exposes the folding methods and will helpfully tell you the default keyboard shortcuts they are assigned to.
By default you press Ctrl+K, then Ctrl+1 to fold all subroutines. Then, to unfold all I would press Ctrl+K then Ctrl+J.
If this does not work for your file type / syntax, try Ctrl+K, Ctrl+2 and above to see if another folding level works for you.
Unfortunately for me, the proprietary code I work with required custom syntax and a custom function written in our in-house package to fold all of our functions (a new sublime command written in python and installed as a package).