Yes, you need to use expert mode. There are a number of threads on the subject, like this one: https://www.mybmwi3.com/forum/viewtopic.php?f=11&t=5646&start=10
I think the 3rd button can easily be set up to do both the trunk and frunk; as each button is supposed to allow for 3 functions, within a Short Mid or Long press.
My 3rd is set up Short(noaction) Mid(Trunk) Long(Frunk). as I could not get the short press to work properly.
