The ionising power of α, β and γ-rays is in the decreasing order:
The α particles are a helium nucleus and the most heavy of the three so hits more particles on its travels so can’t travel as far but ionizes more along its way.
The β particles are highly energetic electrons that, being smaller than α particles, can travel further but ionize less.
The γ particles are highly energetic photons that rarely interact with matter so can travel very far and ionize the least.