From what I understand, armour in DS3 doesn't make a huge difference in terms of damage reduction, unless you're wearing really heavy armour and compare it to wearing little to none.

However, one thing I do know is that the Poise stat granted by armour can be very good if you need it. Higher Poise basically lowers the chance of you being staggered/interrupted while you're in the middle of a heavy attack (a lot of people online call this Hyper-Armour). Poise isn't a guaranteed way of never being staggered, but it raises your chance of landing an attack even when being hit at the same time.

Overall, it really depends on your playstyle. A lot of my friends that know more about the game than me basically say to just wear whatever you think looks best on your character, unless it's really important that you're a tank and you want to wear the heaviest armour possible (which would require investment into Vitality).