If you compare chain +3 and plate +1, chainmail is better 'cause the +3 is also used to saving throws vs physical stuff (fireballs, lightning bolts etc..).
If both have equal amounts of AC values at your char screen, note that Plate is better because it has bonus of -1 towards slashing AC over chain mail.
I guess that chain mail has penalty of +2 AC vs Blunt weapons.
So overall; Plate +1 -> Better than -> Chain Mail +3
Because; its -3 Slash AC Bonus -> Better than -> +2 Blunt AC penalty, -2 Slash AC Bonus.
Note: Check the character sheet when you wear armors (you'll need to scroll down a bit to see individual AC modifiers if there are any), to see what addition penalty or bonuses the Armor puts on the character.