The arrangement of elements in a group of three was done by Döbereiner.

Explanation

**Step-by-Step Text Solution:**

1. **Understanding the Statement**: The question states that the arrangement of elements in a group of three was done by Döbereiner. We need to determine if this statement is true or false.

2. **Identifying Döbereiner's Contribution**: Johann Wolfgang Döbereiner was a German chemist who is known for his work in the early 19th century. He proposed the concept of "triads," which are groups of three elements.

3. **Defining Triads**: Döbereiner arranged elements into triads based on their atomic masses. In each triad, the elements were arranged in increasing order of their atomic masses.

4. **Key Observation**: One of the significant observations made by Döbereiner was that the atomic mass of the middle element in each triad was approximately equal to the average of the atomic masses of the other two elements in the group.

5. **Conclusion**: Since Döbereiner indeed arranged elements in groups of three (triads) based on their atomic masses, we can conclude that the statement is true.

**Final Answer**: The statement is true.

---