As per my understanding, Data Cursor's data tips is not available in "bar3" in R2018b. The data tips in R2018a and earlier would give you data tips at the vertex of the rectangular prism, instead of giving the value passed in data like, for example, "bar" does.
Snippet of code is shown below which help you to show the data tips at the vertex of the 3D bars.
Z = count(1:10,:);
fh = figure;
bargraphs = bar3(Z);
for i = 1:numel(bargraphs)
set(hggetbehavior(bargraphs(i), 'Datacursor'), 'Enable', true);
setinteractionhint(bargraphs(i), 'DataCursor', true);
dcm = datacursormode(fh);
When the above code Runs, figure window will be populated and you also need to enable the Data tip in this Figure Window. Following steps will help to enable the Data tip.
In the manu bar, Tool>Data Tip (enable)
Output (along with the data tip) for the above code snippet is as shown
Notice that this workaround works for regular "bar3" and "grouped" "bar3". However, when you use a datatip, it selects the bar vertices, rather than the top-center.
Hope this helps..!