Can I use all 3 Alter of Memories of the Proc of one unit dying?

Yes, you can trigger all three copies of Altar of Memories when a single friendly unit dies.

Ruling

When a friendly unit dies, each copy of Altar of Memories on your battlefield sees the event and triggers independently. Because each Altar of Memories has a "you may" triggered ability, you choose whether or not to place each individual trigger onto the chain.

Sequence

1. Inciting Event: A friendly unit dies.
2. Triggering: All three Altar of Memories are on the board and satisfy their condition ("When a friendly unit dies").
3. Placement: You choose to place each of the three triggers onto the chain.
4. Resolution: You resolve each trigger one by one (following LIFO - Last-In, First-Out). For each trigger:
   • You exhaust that specific Altar of Memories.
   • You draw 1 card.
   • You put a card from your hand on the top or bottom of your Main Deck.

Nuances

• Independent Triggers: Each Altar of Memories is a separate game object with its own triggered ability. They do not share a limit on how many times they can trigger per death event.
• "You May": Because the ability says "you may," you are not forced to trigger them. You can choose to trigger all three, two, one, or none at all.
• Exhausting: You must exhaust each Altar of Memories individually as part of the resolution of its specific trigger. If you have already exhausted one for another effect, you cannot use that specific copy again until it is readied.

Note: I could not fully verify this ruling against the retrieved rules.