In every cell of a table with $n$ rows and $m$ columns is written one of the letters $a$, $b$, $c$. Every two rows of the table have the same letter in at most $k\geq 0$ positions and every two columns coincide at most $k$ positions. Find $m$, $n$, $k$ if
\[\frac{2mn+6k}{3(m+n)}\geq k+1\]
1. **Understanding the Problem:**
   We are given a table with \( n \) rows and \( m \) columns, where each cell contains one of the letters \( a \), \( b \), or \( c \). The problem states that any two rows have the same letter in at most \( k \) positions, and any two columns coincide in at most \( k \) positions. We need to find \( m \), \( n \), and \( k \) such that the inequality
   \[
   \frac{2mn + 6k}{3(m+n)} \geq k + 1
   \]
   holds.

2. **Double Counting the Triplets:**
   Consider the triplets \((C_1, C_2, R)\), where \(C_1\) and \(C_2\) are columns and \(R\) is a row such that the entries in \(C_1\) and \(C_2\) coincide in the position corresponding to \(R\). Let \(T\) be the number of such triplets.

3. **Upper Bound on \(T\):**
   Since any two columns coincide in at most \(k\) positions, the number of triplets is at most:
   \[
   T \leq \frac{m(m-1)}{2} k
   \]
   This is because there are \(\frac{m(m-1)}{2}\) pairs of columns, and each pair can coincide in at most \(k\) rows.

4. **Lower Bound on \(T\):**
   For a fixed row, let \(n_i\) be the number of cells containing the letter \(i\) (where \(i \in \{a, b, c\}\)). The number of pairs of columns that coincide in this row is:
   \[
   \sum_{i} \frac{n_i(n_i-1)}{2}
   \]
   Using the Cauchy-Schwarz inequality, we can find a lower bound for this sum. Since there are \(m\) columns and each cell contains one of three letters, we have:
   \[
   \sum_{i} n_i = m
   \]
   By Cauchy-Schwarz:
   \[
   \left( \sum_{i} n_i \right)^2 \leq 3 \sum_{i} n_i^2 \implies m^2 \leq 3 \sum_{i} n_i^2 \implies \sum_{i} n_i^2 \geq \frac{m^2}{3}
   \]
   Therefore:
   \[
   \sum_{i} \frac{n_i(n_i-1)}{2} \geq \frac{1}{2} \left( \frac{m^2}{3} - m \right) = \frac{m^2 - 3m}{6}
   \]
   Summing over all \(n\) rows, we get:
   \[
   T \geq \frac{(m^2 - 3m)n}{6}
   \]

5. **Combining the Bounds:**
   We have:
   \[
   \frac{m(m-1)}{2} k \geq T \geq \frac{(m^2 - 3m)n}{6}
   \]
   Simplifying the inequality:
   \[
   \frac{m(m-1)}{2} k \geq \frac{(m^2 - 3m)n}{6}
   \]
   Multiplying through by 6:
   \[
   3m(m-1)k \geq (m^2 - 3m)n
   \]
   Simplifying:
   \[
   3m^2k - 3mk \geq m^2n - 3mn
   \]
   Rearranging terms:
   \[
   3m^2k - m^2n \geq 3mk - 3mn
   \]
   Factoring out common terms:
   \[
   m^2(3k - n) \geq 3m(k - n)
   \]
   Dividing by \(m\) (assuming \(m \neq 0\)):
   \[
   m(3k - n) \geq 3(k - n)
   \]
   Dividing by 3:
   \[
   m(k - \frac{n}{3}) \geq k - n
   \]

6. **Analyzing the Given Inequality:**
   Given:
   \[
   \frac{2mn + 6k}{3(m+n)} \geq k + 1
   \]
   Simplifying:
   \[
   \frac{2mn + 6k}{3(m+n)} \geq k + 1
   \]
   Multiplying through by \(3(m+n)\):
   \[
   2mn + 6k \geq 3(k+1)(m+n)
   \]
   Expanding:
   \[
   2mn + 6k \geq 3km + 3kn + 3m + 3n
   \]
   Rearranging terms:
   \[
   2mn + 6k \geq 3km + 3kn + 3m + 3n
   \]
   Simplifying:
   \[
   2mn + 6k - 3km - 3kn \geq 3m + 3n
   \]
   Factoring out common terms:
   \[
   2mn - 3kn - 3km + 6k \geq 3m + 3n
   \]

The final answer is \( \boxed{ m, n, k } \) such that the inequality holds.