I used the item below for a watchdog in OH2, but it doens’t appear that OH3 supports the COUNT aggregation function. What is the recommended alternative? I’m struggling on how I should migrate this functionality.
Group:Number:COUNT("NULL") grp_WD "Watchdog Count [%.1f]"
Not just my and subsequent posts to this were lost by the Feb 6-7 service interruption, but I even had to recreate my account. 
For the record (i.e., for anyone encountering the same issue), I posted that I’d found I could paste text such as the above into the “Add Items from Textual Definitions” option after clicking on the “+” circle bottom-right in the Items page. I also noted I saw no evidence of the COUNT function being active on the group on the GUI page for the Item but it seemed to work for me.
Another post mentioned the quotes around NULL might not be right.
msexton187 subsequently reported that “As for the COUNT function, it does work after being created from a text file. I had to change “NULL” to NULL, but that was it.”