What are the three main types of radiation in order of increasing penetrating abilities?
Alpha decay is the least penetrating. This is because the Alpha particle produced during the decay process is relatively large and low in energy thus it is easily 'blocked' and does not travel far through space.
Beta (plus and minus decay) particles are more penetrating than alpha decay as they are higher in energy and have a smaller mass / size (a high energy electron (minus decay) or a high energy positron (plus decay)).
Gamma Radiation produces the most penetrating form of radiation. No actual particles are involved, but a very high energy wave that, due to it's size and energy level, is easily able to penetrate through molecules and substances. Infact, it can take up to 2 metres of solid concrete or 50cm of solid lead to stop a gamma radiation wave (Gamma Ray)!