The strangeness is not conserved in the decay of the positive Kaon into 3 pions. so this decay should be a weak process but on the other hand parity is conserved in this decay. Kaon has odd parity and overall parity of 3 pions is also odd, so this should be strong or em process as parity is conserved in strong and em processes. Weak processes violate Parity.
Strangeness isn't conserved->Weak decay.
As for parity conservation, weak decay CAN change parity, it doesn't have to.
Basically, knowing that something IS conserved doesn't give you any information, but knowing that it isn't conserved may give you some. (e.g., electrical charge being conserved means nothing, but it being not conserved means this decay is impossible. Strangeness being conserved means nothing, but it being not conserved means weak decay)