if you don't get a helm buffer popping up, it could mean you have not turned helm-mode on (e.g. M-x helm-mode). That redefines completing-read in a helm style, and should act and feel like helm when you insert a ref link. If you don't have that on, then you will see the older completing read behavior where you have to press tab to see the candidates etc.
in v3, you can customize the insert functions any way you want, so if you want to use the old v2 functions and have them around, there should be no issue with that. I think if you set `org-ref-insert-ref-function` to the function you want, it should just work the way you want.
John
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Professor John Kitchin (he/him/his)
Doherty Hall A207F
Department of Chemical Engineering
Carnegie Mellon University
Pittsburgh, PA 15213
412-268-7803