Aligned multline

Here is what I would do (require mathtools package)

\begin{align}
   \MoveEqLeft[3]  (a+b+c+d+e+f) \\ 
    ={}& a+b+c+d+e+f+g+h+i+j+k+l+m+n\\
\begin{split}
    ={}& a+b+c+d+e+f+g+h+i+j+k+l+m+n \\
    &  +o+p+q+r+s+t+u+v+w+x+y+z+1+2+3+4
\end{split}\\
    ={}& a+b+c+d+e+f+g+h+i+j+k+l+m+n+o+p
\end{align}

I would not want to start using multline effects in it, ends up looking strange. The above sort of follows the guides set out by the AMS.

Here is a version without magic numbers. It uses the mathtools to make the first line appear to have zero width.

\begin{align}
    \mathclap{(a+b+c+d+e+f)} & \notag \\
    &= a+b+c+d+e+f+g+h+i+j+k+l+m+n \\
    &= a+b+c+d+e+f+g+h+i+j+k+l+m+n \notag \\
    & \quad +o+p+q+r+s+t+u+v+w+x+y+z+1+2+3+4 \\
    &= a+b+c+d+e+f+g+h+i+j+k+l+m+n+o+p
\end{align}