Find all solutions $x,y,z$  in the positive integers of the equation $$3^x -5^y = z^2$$
1. Consider the given equation:
   \[
   3^x - 5^y = z^2
   \]
   We need to find all solutions \((x, y, z)\) in the positive integers.

2. First, let's analyze the equation modulo 3:
   \[
   3^x - 5^y \equiv z^2 \pmod{3}
   \]
   Since \(3^x \equiv 0 \pmod{3}\) for \(x \geq 1\), we have:
   \[
   -5^y \equiv z^2 \pmod{3}
   \]
   Note that \(5 \equiv 2 \pmod{3}\), so:
   \[
   -2^y \equiv z^2 \pmod{3}
   \]
   The possible values of \(2^y \pmod{3}\) are:
   \[
   2^1 \equiv 2 \pmod{3}, \quad 2^2 \equiv 1 \pmod{3}, \quad 2^3 \equiv 2 \pmod{3}, \quad 2^4 \equiv 1 \pmod{3}, \ldots
   \]
   Therefore, \(2^y \equiv 2 \pmod{3}\) if \(y\) is odd, and \(2^y \equiv 1 \pmod{3}\) if \(y\) is even. Thus:
   \[
   -2 \equiv z^2 \pmod{3} \quad \text{if } y \text{ is odd}
   \]
   \[
   -1 \equiv z^2 \pmod{3} \quad \text{if } y \text{ is even}
   \]
   Since \(z^2 \equiv 0 \text{ or } 1 \pmod{3}\), \(y\) must be even. Let \(y = 2k\).

3. Substitute \(y = 2k\) into the original equation:
   \[
   3^x - 5^{2k} = z^2
   \]
   We can rewrite this as:
   \[
   3^x = z^2 + 5^{2k}
   \]

4. Next, consider the equation modulo 5:
   \[
   3^x \equiv z^2 \pmod{5}
   \]
   The possible values of \(3^x \pmod{5}\) are:
   \[
   3^1 \equiv 3 \pmod{5}, \quad 3^2 \equiv 4 \pmod{5}, \quad 3^3 \equiv 2 \pmod{5}, \quad 3^4 \equiv 1 \pmod{5}, \ldots
   \]
   Therefore, \(3^x \equiv 1 \pmod{5}\) if \(x\) is a multiple of 4. Let \(x = 4m\).

5. Substitute \(x = 4m\) into the original equation:
   \[
   3^{4m} - 5^{2k} = z^2
   \]

6. Now, we use Zsigmondy's Theorem, which states that for any \(a^n - b^n\) (with \(a > b > 0\) and \(n > 1\)), there exists a prime \(p\) that divides \(a^n - b^n\) but does not divide \(a^k - b^k\) for any \(k < n\), except for a few specific cases. Applying this theorem to \(3^{4m} - 5^{2k}\), we see that for \(m > 1\) and \(k > 1\), there exists a prime \(p\) dividing \(3^{4m} - 5^{2k}\) but not dividing \(3^4 - 5^2\).

7. Therefore, we need to check small values of \(m\) and \(k\). For \(m = 1\) and \(k = 1\):
   \[
   3^4 - 5^2 = 81 - 25 = 56
   \]
   which is not a perfect square.

8. For \(m = 1\) and \(k = 2\):
   \[
   3^4 - 5^4 = 81 - 625 = -544
   \]
   which is not a perfect square.

9. For \(m = 2\) and \(k = 1\):
   \[
   3^8 - 5^2 = 6561 - 25 = 6536
   \]
   which is not a perfect square.

10. For \(m = 1\) and \(k = 1\):
    \[
    3^2 - 5^2 = 9 - 25 = -16
    \]
    which is not a perfect square.

11. Finally, for \(x = 2\) and \(y = 2\):
    \[
    3^2 - 5^2 = 9 - 25 = -16
    \]
    which is not a perfect square.

12. Therefore, the only solution is:
    \[
    (x, y, z) = (2, 2, 4)
    \]

The final answer is \( \boxed{ (2, 2, 4) } \).