For three or more electrons we will need a symmetrical figure. Three electrons will form a equilateral triangle, four a square, etc. This is a one-dimensional problem with all momenta measured along the ling connecting the particles. Initial condition since the energy of the alpha is much less than the rest mass, we can use the non-relativistic formula for the kinetic energy . Also, in alpha scattering with atoms we can ignore the scattering with the electrons in first approximation.
Longest wavelength is smallest energy difference.