That one is pretty self-explanatory.
You can find the displays in the workshop menu under "Decorations > Displays"
With a power armor you need to exit your armor inside the display and then activate the display (needs to be powered).
With the armor, build a mannequin, transfer a piece of armor and equip it (like with a follower)
With the weapon, build the weapon stand and choose a weapon to display.
Once you've done all of the 3, you'll get your achievement.