Find all sequences $a_1, a_2, a_3, \dots$ of real numbers such that for all positive integers $m,n\ge 1$, we have
\begin{align*}
a_{m+n} &= a_m+a_n - mn \text{ and} \\
a_{mn} &= m^2a_n + n^2a_m + 2a_ma_n. \\
\end{align*}
1. **Initial Conditions and Simplification:**
   We start by analyzing the given conditions for the sequence \( \{a_n\} \):
   \[
   a_{m+n} = a_m + a_n - mn
   \]
   \[
   a_{mn} = m^2 a_n + n^2 a_m + 2a_m a_n
   \]

2. **Finding \(a_1\):**
   Let's set \( m = n = 1 \) in the second equation:
   \[
   a_1 = 1^2 a_1 + 1^2 a_1 + 2a_1 a_1
   \]
   Simplifying, we get:
   \[
   a_1 = 2a_1 + 2a_1^2
   \]
   \[
   0 = 2a_1 + 2a_1^2 - a_1
   \]
   \[
   0 = 2a_1^2 + a_1
   \]
   \[
   a_1(2a_1 + 1) = 0
   \]
   Thus, \( a_1 = 0 \) or \( a_1 = -\frac{1}{2} \).

3. **Case 1: \(a_1 = 0\):**
   We use the first equation to find a general form for \(a_n\):
   \[
   a_{n+1} = a_n + a_1 - n = a_n - n
   \]
   Using induction, assume \( a_n = -\frac{n(n-1)}{2} \):
   \[
   a_{n+1} = -\frac{n(n-1)}{2} - n = -\frac{n^2 - n + 2n}{2} = -\frac{n^2 + n}{2} = -\frac{(n+1)n}{2}
   \]
   Thus, \( a_n = -\frac{n(n-1)}{2} \) is a solution.

4. **Case 2: \(a_1 = -\frac{1}{2}\):**
   Again, use the first equation to find a general form for \(a_n\):
   \[
   a_{n+1} = a_n + a_1 - n = a_n - \frac{1}{2} - n
   \]
   Using induction, assume \( a_n = -\frac{n^2}{2} \):
   \[
   a_{n+1} = -\frac{n^2}{2} - \frac{1}{2} - n = -\frac{n^2 + 2n + 1}{2} = -\frac{(n+1)^2}{2}
   \]
   Thus, \( a_n = -\frac{n^2}{2} \) is a solution.

5. **Verification:**
   We need to verify that both solutions satisfy the second condition:
   - For \( a_n = -\frac{n(n-1)}{2} \):
     \[
     a_{mn} = -\frac{mn(mn-1)}{2}
     \]
     \[
     m^2 a_n + n^2 a_m + 2a_m a_n = m^2 \left(-\frac{n(n-1)}{2}\right) + n^2 \left(-\frac{m(m-1)}{2}\right) + 2 \left(-\frac{m(m-1)}{2}\right) \left(-\frac{n(n-1)}{2}\right)
     \]
     Simplifying, we find both sides match.
   - For \( a_n = -\frac{n^2}{2} \):
     \[
     a_{mn} = -\frac{(mn)^2}{2}
     \]
     \[
     m^2 a_n + n^2 a_m + 2a_m a_n = m^2 \left(-\frac{n^2}{2}\right) + n^2 \left(-\frac{m^2}{2}\right) + 2 \left(-\frac{m^2}{2}\right) \left(-\frac{n^2}{2}\right)
     \]
     Simplifying, we find both sides match.

The final answer is \( \boxed{ a_n = -\frac{n(n-1)}{2} } \) or \( a_n = -\frac{n^2}{2} \).