When Isaac uses a Card, Pill, or Rune, he also uses a copy of the previous three Cards/Pills/Runes he used after picking up Echo Chamber.
Previously used cards, pills, and runes are effectively used again, meaning items that modify the effects of cards, pills, or runes will also modify the effects that Echo Chamber activates.
As a result, previously taken golden pills and blank runes will activate a random effect each time Echo Chamber activates them.
If Isaac uses cards that convert/destroy pickups (such as Ace of Spades, Black Rune, IX - The Hermit?, etc.), any other cards Echo Chamber activates that generate pickups have a high chance to have their pickups converted.
It is advised to avoid using cards that teleport Isaac, as he will be teleported to a specific room the next three times he uses a consumable. This effectively negates any current-room effects from consumables, such as XV - The Devil or Rune of Algiz, during that period, as Isaac is teleported after they activate.
Similarly, it is also advised to avoid using unidentified pills, as Isaac runs the risk of activating either a stat down or Telepills for the next three consumables he uses.
When obtained on Tainted Lazarus, Echo Chamber will only duplicate the last three cards or pills previously used on the Lazarus state with Echo Chamber. As a result, teleport cards or Telepills can be freely used on the Lazarus state without Echo Chamber.
If an item is somehow removed and reobtained during the run (such as when obtained from Lemegeton or Modeling Clay), the last three consumable types used during the time Isaac had Echo Chamber will be remembered. Genesis and Victory Laps will erase all of the remembered consumable types, however.
A card, rune, or pill must be consumed for the effect of Echo Chamber to trigger. That means using Blank Card, Placebo, or Clear Rune will not trigger Echo Chamber.
While Cracked Key will activate the effect of Echo Chamber and trigger the last three consumables used, using a different consumable near a wall that a red room could be created on will not create a red room, even if a Cracked Key has been used before.
? Card + Ancient Recall / Vurp Pill + R Key: The ? Card effect can be used in conjunction with R Key without consuming it, allowing the player to in theory (with the help of Clicker) obtain completion marks for up to four additional routes and characters in a single run.
False PHD: Taking benign pills (e.g. puberty) will give black hearts or damage ups for each of the pills that are activated. Caution should be exercised when using this synergy, since the last three pills taken will be converted to their bad counterparts, and one stat down can ruin the synergy.
YAZ7 ZEP9 First secret room (left of room before boss)
1V1X PNQ6 Eden starts with this item
This is possibly a reference to the card Eye of the Storm from Magic: the Gathering, as the item resembles the art of the card and both result in previous effects being reused when a new one is activated.
This is also possibly a reference to the card Echo Form from 'Slay the Spire', which will duplicate the card effect of the first card played per turn.
Before the 1.7.7 patch, Echo Chamber was able to copy effect off all previously used cards and pills, allowing player to get infinite cards and pills with Ancient Recall.