Hi. I recently wondered if it makes a difference, if you only have thorns 3 on the chestplate, or on the whole armor.
As far as I know, yes- whether you really do more damage… I'm not 100% sure but tend to yes.
Definitely you have more wear on your armor parts - so if you play 100% survival you should try to "repair" the armor before you take "thorns" with you.
(Since constant repairs with the anvil become too expensive after a few repetitions)