You only can use any given infusion once
You are referring to the 9th level Armor Modifications feature of the Armorer subclass, which states:
You learn how to use your artificer infusions to specially modify your Arcane Armor. That armor now counts as separate items for the purposes of your Infuse Items feature: armor (the chest piece), boots, helmet, and the armor's special weapon. Each of those items can bear one of your infusions, and the infusions transfer over if you change your armor's model with the Armor Model feature. In addition, the maximum number of items you can infuse at once increases by 2, but those extra items must be part of your Arcane Armor.
This will allow you to use multiple infusions with your armor, one for each piece. However, there is this general rule for infusions (p. 13 Tasha's Cauldron of Everything):
You can infuse more than one nonmagical object at the end of a long rest; the maximum number of objects appears in the Infused Items column of the Artificer table. You must touch each of the objects, and each of your infusions can be in only one object at a time.
So even if your armor counts as several objects that can be infused, you still can use no infusion twice, because each infusion can only be at one object at a time.
That means you can only use the Enhanced Defense infusion, that grants +1 to AC (or +2 after level 10) in one of these armor pieces, even when you have the Armor Modification feature. You cannot use it three times to get triple that bonus.
However, you can use other armor-enhancing infusions along with it, such as Repulsion Shield or Resistant Armor, on other pieces of Armor or your Shield.