A spell +2 AC and an amulet +1 AC stack for a total of +3 AC. Two effects of +2 or lower to the same destination can stack.
A spell +3 AC and a ring +2 AC do not stack. You get only +3 AC total. The +3 effect suppresses the smaller one.
You cannot run a potion +2 AC, a spell +2 AC, magic armor +2 AC, and a magic sword +1 AC all to AC. That is 4 effects trying to reach the same destination. Only 2 may apply. If more than 2 effects would apply, the creature chooses which 2 apply. Max allowed is 2 magical effects per stat if none are +3 or higher.
A potion of +4 Constitution and a spell +3 Constitution do not give +7 Constitution. You take only +4 Constitution, since a modifier of +3 or higher blocks other stacking on that destination.