For the 3rd memory, you must discover the recipe for the “ice bomb”. To do this, you must perform synthesis and obtain water. Before we get there though, you are going to have to unlock some more recipes.
Note: Depending on how much you’ve gathered and synthesized prior to this, you may not have to complete all of these steps to finish the 3rd memory. The biggest part of this memory is mostly unlocking and synthesizing new recipes.
First, you need to make an Ingot using the recipe that you learned from Plachta’s previous memory. The quality of the Ingot does not matter much, so just use whatever you have.
Next, you need to leave the town and go to the Blessed Forest and gather materials until you find a Beehive. This will unlock the recipe for Honey. You can also gather Red Uni’s at night here to unlock the Neutralizer R recipe.
Return to the atelier and make some Honey using the Beehive and some water. You should also make some Neutralizer R using any water and plant materials that you have.
After making Honey, Oskar will visit you at the atelier and ask you to make some Simple Sweets for him. You need to use Honey, Flour and any other sweet material, such as Sugar or Fruit. You can unlock the recipe for Ramel Flour by visiting Merbert Farm and investigating the wheat field.
Before making Simple Sweets, go to the Forgotten Nursery and gather materials there. You should find some Silver Web, which will unlock the recipe for Gold Thread.
Go back to the atelier and make some Simple Sweets for Oskar. However, you need to make them with a Salty effect. To do this, you need to make the bottom bar 25-49. After making Salty Simple Sweets, go to the Street and give them to Oskar. You will also meet Harol, a clockmaker who is interested in Plachta’s book.
Now, you need to make some Cloth using Animal Fur and Neutralizer R. You can get Animal Fur by fighting Chimera Beasts at Merbert Farm during night time. You need at least two Animal Fur items to make Cloth. The Chimera Beast is only necessary to discover the recipe, after that you can also use Monster Feathers, which you can gather from Blessed Forest.
After making Cloth, you will get an idea for Meister Mittens, which are gloves that enhance your alchemy skills. To make them, you need Cloth, Gold Thread and Ingot. You should have enough materials to make them, but if not, you can gather more from the previous areas.
After making Meister Mittens, equip them to Sophie and go back to Merbert Farm during day time. There, you will find a place called Black Cavern. Enter it and explore it until you find a large crystal called Hakurei Stone. This will unlock the recipe for Ice Bomb.
Make some Ice Bombs using Hakurei Stone, Water, and either Neutralizer B or Neutralizer R. You will also unlock a new combat feature called Chain Burst, which allows you to use multiple actions in a row for extra damage.