If alpha, beta and gamma rays carry same momentum, which has the longest wavelength?

None, all have same wavelength

On the basis of dual nature of light, Lious de-Broglie suggested that the dual nature is not only of light, but each moving material particle has the dual nature. He assumed a wave to be associated with each moving material particle which is called the matter wave. The wavelength of this wave is determined by the momentum of the particle. If p is the momentum of the particle, the wavelength of the wave associated with it is

$λ =\frac{h}{p}$

Where h is Plank’s constant.

Since, it is given that, alpha, beta and gamma rays carry same momentum, so they will have same wavelength.