Thresh = Long windup hook. Can't pull through walls. Engages by bringing himself into the middle of the enemy team followed by a slight knockup and slow. If things go bad and he needs to disengage he has nothing to do so.
Blitzcrank = Fast grab. Can pull through walls. Engages by bringing one enemy into the middle of his team followed by a large knockup and a silence. If things go bad and he has to disengage he has a huge MS steroid, a built-in shield, and the enemy will be silenced from his ult.
Blitzcrank and Malphite have a bigger presence in games.
You can be hugging you inner Mid turret and blitz walks up to a jungle wall and pulls you over it and laughs as you're dead before you realize what happened. His Ult has a passive so he doesn't even need to use it to hurt random champions in team fights, then it has a Silence as big as Amumu's ult stun.
Whereas Malphite can initiate with his ult and if he has enough AP laugh as you are hurt badly then Do Movement speed vamp(He takes half of your Movement speed away and adds it to his for about 5 seconds) and chases you down until he's smacked you to death.
Where as Thresh can focus one champion and it takes time for him to finish that single one, the other 2 can either disable a whole enemy team or nuke a single champion and then focus on the remaining team. Thresh is a joke compared to these 2 when you compare their kits.
Xbox 360 GamerTag: HolyKnight22 PSN: CrimsonDivide