Default under 10.11 (on this 2015 Macbook) is Force in all applications. It doesn’t work in Scrivener. So the lookup can only be activated via key shortcut or right-click context menu as you say.
Re-enabling 3-finger-tap seems to disable Force in all applications, but the means I don’t have to go through the context menu or use the key shortcut.
So I guess I’ll switch force touch back on as I love it in other applications. For now, in Scrivener, I’ll train myself to use the keyboard short cut