To sum up, in SIERRA :
3. sometimes I will get no popup or any message of any kind (per joemikeb #42524)
Where is your cursor when that happens?
Is it hovering over a word or somewhere/thing else?
More: Do you get a pop-up with your cursor hovering over
anything but text?
Still more: I've always assumed that a three-finger tap has been no more than an alternative to command-control-D...the original way to access dictionary/thesaurus.
Is that correct, or do the two somehow differ (other than as respects the new functionality in Sierra).