A group of $n$ people play a board game with the following rules:
1) In each round of the game exactly $3$ people play
2) The game ends after exactly $n$ rounds
3) Every pair of players has played together at least at one round
Find the largest possible value of $n$
1. Let's denote the group of \( n \) people as \( P_1, P_2, \ldots, P_n \).
2. Each round involves exactly 3 players, and there are \( n \) rounds in total.
3. We need to ensure that every pair of players has played together at least once.

To find the largest possible value of \( n \), we need to consider the combinatorial constraints. Specifically, we need to ensure that every pair of players is included in at least one of the rounds.

4. The total number of pairs of players in a group of \( n \) people is given by the binomial coefficient:
   \[
   \binom{n}{2} = \frac{n(n-1)}{2}
   \]

5. Each round involves 3 players, and each round can cover exactly 3 pairs of players. Therefore, the total number of pairs covered in \( n \) rounds is:
   \[
   3n
   \]

6. For every pair to be covered at least once, the total number of pairs must be less than or equal to the total number of pairs covered in \( n \) rounds:
   \[
   \frac{n(n-1)}{2} \leq 3n
   \]

7. Simplifying the inequality:
   \[
   \frac{n(n-1)}{2} \leq 3n
   \]
   \[
   n(n-1) \leq 6n
   \]
   \[
   n^2 - n \leq 6n
   \]
   \[
   n^2 - 7n \leq 0
   \]
   \[
   n(n - 7) \leq 0
   \]

8. The inequality \( n(n - 7) \leq 0 \) holds for \( 0 \leq n \leq 7 \).

9. We need to check if \( n = 7 \) is feasible. If \( n = 7 \), then we have 7 rounds, and each round involves 3 players. We need to ensure that every pair of players is included in at least one round.

10. For \( n = 7 \), the total number of pairs is:
    \[
    \binom{7}{2} = \frac{7 \cdot 6}{2} = 21
    \]
    The total number of pairs covered in 7 rounds is:
    \[
    3 \cdot 7 = 21
    \]
    Since the number of pairs matches, it is possible to cover all pairs.

11. Therefore, the largest possible value of \( n \) is 7.

The final answer is \( \boxed{7} \).