Hi @FrylockBeedrillite, Pinsirite and Heracronite can be obtained once the player has a Mega Bracelet. Once obtaining the Mega Bracelet the player will need to win the Bug catching contest 3 times for the first stone, further 3 times for the second and another 3 times for the last one. The wins do not need to be in a row and they are randomly. These are also a 1 time only reward.
this is from prowiki it says that wins do not need to be consecutive