I have been doing some investigating as to how to display a hidden (display none) flex item. Is the best approach to use toggle() instead of changing display?
I have three hidden DIV’s, and only one will be displayed based on user input
flex items
A
B
C
D or E or F
G
H
I
J
etc.
Seeing your actual HTML and CSS rather
than text would be much more beneficial
for members, as it would then allow them
to see the problem and possibly resolve it. 
Use JS to toggle the class on the element concerned.
Then use CSS to show it.
e.g.
.invisible{display:none;}
.visible{display:flex;}