#1 is an attack and can be modified in some ways (such as spells), but not by your armor.
#2 and #3, however, are not attacks and there is nothing I know of that could increase their damage.
Magic Armor (DMG p152) will not increase the damage (or attack roll) for #1. All it does is provide an AC bonus.
Magic Weapon (DMG p213) is meant to be used with weapons. Battleragers gain the ability to use spiked armor as a weapon (SCAG p121). This does not actually make spiked armor a weapon.
So by RAW, there is nothing that the spiked armor can do to help your attack. However, the DM is free to create magic items as treasure as they see fit (unless you are playing in Adventurer's League).
Magic Armor is rare (+1), very rare(+2), or legendary (+3).
Magic Weapons are uncommon(+1), rare (+2), or very rare(+3).
It might be reasonable for a DM to create custom spiked armor with magic spikes which are a Magic Weapon. It might also be reasonable to combine the 2 magic items by increasing the rarity a step. So maybe a Legendary Spiked Armor which provides +1 AC and counts as a +2 weapon for attacks and damage (the DM would need to determine whether the +2 affects #2 and #3 or just #1).