diff --git a/3/input b/3/input
new file mode 100644
index 0000000..7565e91
--- /dev/null
+++ b/3/input
@@ -0,0 +1,300 @@
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diff --git a/3/solution.mjs b/3/solution.mjs
new file mode 100644
index 0000000..045f8c5
--- /dev/null
+++ b/3/solution.mjs
@@ -0,0 +1,66 @@
+import { readFileSync } from 'node:fs';
+
+const input = readFileSync('input', 'utf-8');
+
+function priorityForItem(item) {
+  const c = item.charCodeAt(0);
+  return c - (c >= 97 ? 96 : 38)
+}
+
+const rucksacks = input
+  .split('\n')
+  .filter(line => {
+    if (!line) return false;
+    if (line.length % 2 !== 0) {
+      throw new Error('Rucksack with unequal compartments');
+    }
+    return true;
+  })
+  .map(line => line.split(''))
+  .map(rucksack => ([
+    // compartment 1
+    rucksack.slice(0, rucksack.length / 2),
+    // compartment 2
+    rucksack.slice(rucksack.length / 2)
+  ]));
+
+const sumOfPriorities = rucksacks
+  .map(([compartment1, compartment2]) => {
+    const commonItems = [...new Set(
+      compartment1.filter(item => compartment2.includes(item))
+    )];
+    if (commonItems.length !== 1) {
+      throw new Error('Unexpected number of common items');
+    }
+    return priorityForItem(commonItems[0]);
+  })
+  .reduce((total, priority) => total + priority, 0);
+
+console.log(`Sum of priorities of common items: ${sumOfPriorities}`);
+
+const elfRucksackGroups = rucksacks
+  .reduce((groups, rucksack) => {
+    let group = groups[groups.length - 1];
+    if (group.length >= 3) {
+      group = [];
+      groups.push(group);
+    }
+    group.push(rucksack);
+    return groups;
+  }, [[]]);
+
+const commonItemsInGroup = elfRucksackGroups
+  .map(group => group
+    .reduce((itemsA, itemsB) => {
+      itemsA = [...new Set(itemsA.flat())];
+      itemsB = [...new Set(itemsB.flat())];
+      return itemsA.filter(item => itemsB.includes(item))
+    })
+  )
+
+const badgePriorities = commonItemsInGroup
+  .map(items => priorityForItem(items[0]))
+  .reduce((total, priority) => total + priority, 0)
+
+console.log(`Sum of badge priorities: ${badgePriorities}`);
+