Develop necessary and sufficient conditions that the equation \[ \begin{vmatrix} 0 & a_1 - x & a_2 - x \\ -a_1 - x & 0 & a_3 - x \\ -a_2 - x & -a_3 - x & 0\end{vmatrix} = 0 \qquad (a_i \neq 0) \] shall have a multiple root.

To determine the necessary and sufficient conditions for the equation 

\[
\begin{vmatrix} 
0 & a_1 - x & a_2 - x \\ 
-a_1 - x & 0 & a_3 - x \\ 
-a_2 - x & -a_3 - x & 0 
\end{vmatrix} = 0 \quad (a_i \neq 0)
\]

to have a multiple root, we need to analyze the determinant and its roots.

1. **Set up the determinant:**

   The given determinant is a $3 \times 3$ matrix:

   \[
   \begin{vmatrix} 
   0 & a_1 - x & a_2 - x \\ 
   -a_1 - x & 0 & a_3 - x \\ 
   -a_2 - x & -a_3 - x & 0 
   \end{vmatrix}
   \]

2. **Calculate the determinant:**

   Using the cofactor expansion along the first row, we get:

   \[
   \begin{vmatrix} 
   0 & a_1 - x & a_2 - x \\ 
   -a_1 - x & 0 & a_3 - x \\ 
   -a_2 - x & -a_3 - x & 0 
   \end{vmatrix} = 0 \cdot \begin{vmatrix} 0 & a_3 - x \\ -a_3 - x & 0 \end{vmatrix} - (a_1 - x) \cdot \begin{vmatrix} -a_1 - x & a_3 - x \\ -a_2 - x & 0 \end{vmatrix} + (a_2 - x) \cdot \begin{vmatrix} -a_1 - x & 0 \\ -a_2 - x & -a_3 - x \end{vmatrix}
   \]

   Simplifying the 2x2 determinants:

   \[
   \begin{vmatrix} -a_1 - x & a_3 - x \\ -a_2 - x & 0 \end{vmatrix} = (-a_1 - x)(0) - (a_3 - x)(-a_2 - x) = (a_3 - x)(a_2 + x)
   \]

   \[
   \begin{vmatrix} -a_1 - x & 0 \\ -a_2 - x & -a_3 - x \end{vmatrix} = (-a_1 - x)(-a_3 - x) - (0)(-a_2 - x) = (a_1 + x)(a_3 + x)
   \]

   Substituting back into the determinant expansion:

   \[
   \begin{vmatrix} 
   0 & a_1 - x & a_2 - x \\ 
   -a_1 - x & 0 & a_3 - x \\ 
   -a_2 - x & -a_3 - x & 0 
   \end{vmatrix} = -(a_1 - x)(a_3 - x)(a_2 + x) + (a_2 - x)(a_1 + x)(a_3 + x)
   \]

3. **Simplify the expression:**

   Expanding the terms:

   \[
   -(a_1 - x)(a_3 - x)(a_2 + x) = -(a_1a_3a_2 + a_1a_3x - a_1x^2 - a_3x^2 + x^3)
   \]

   \[
   (a_2 - x)(a_1 + x)(a_3 + x) = a_2a_1a_3 + a_2a_1x + a_2a_3x + a_2x^2 - a_1x^2 - a_3x^2 - x^3
   \]

   Combining like terms:

   \[
   -(a_1a_3a_2 + a_1a_3x - a_1x^2 - a_3x^2 + x^3) + (a_2a_1a_3 + a_2a_1x + a_2a_3x + a_2x^2 - a_1x^2 - a_3x^2 - x^3)
   \]

   Simplifying further:

   \[
   = -a_1a_3a_2 - a_1a_3x + a_1x^2 + a_3x^2 - x^3 + a_2a_1a_3 + a_2a_1x + a_2a_3x + a_2x^2 - a_1x^2 - a_3x^2 - x^3
   \]

   \[
   = (a_2a_1a_3 - a_1a_3a_2) + (a_2a_1x + a_2a_3x - a_1a_3x) + (a_1x^2 + a_3x^2 + a_2x^2 - a_1x^2 - a_3x^2 - x^3 - x^3)
   \]

   \[
   = 0 + (a_2a_1x + a_2a_3x - a_1a_3x) + (a_2x^2 - x^3 - x^3)
   \]

   \[
   = a_2a_1x + a_2a_3x - a_1a_3x + a_2x^2 - 2x^3
   \]

4. **Identify the roots:**

   The roots of the polynomial are given by solving the equation:

   \[
   a_2a_1x + a_2a_3x - a_1a_3x + a_2x^2 - 2x^3 = 0
   \]

   Factoring out common terms:

   \[
   x(a_2a_1 + a_2a_3 - a_1a_3 + a_2x - 2x^2) = 0
   \]

   This implies:

   \[
   x = 0 \quad \text{or} \quad a_2a_1 + a_2a_3 - a_1a_3 + a_2x - 2x^2 = 0
   \]

   Solving the quadratic equation:

   \[
   2x^2 - a_2x + (a_1a_3 - a_2a_1 - a_2a_3) = 0
   \]

   The discriminant of this quadratic equation is:

   \[
   \Delta = a_2^2 - 4 \cdot 2 \cdot (a_1a_3 - a_2a_1 - a_2a_3)
   \]

   For the quadratic equation to have a multiple root, the discriminant must be zero:

   \[
   a_2^2 - 8(a_1a_3 - a_2a_1 - a_2a_3) = 0
   \]

   Simplifying:

   \[
   a_2^2 = 8(a_1a_3 - a_2a_1 - a_2a_3)
   \]

   Therefore, the necessary and sufficient condition for the equation to have a multiple root is:

   \[
   a_2^2 = 8(a_1a_3 - a_2a_1 - a_2a_3)
   \]

The final answer is \( \boxed{ a_2^2 = 8(a_1a_3 - a_2a_1 - a_2a_3) } \)