How to chat from my S3 to my PC?
Well I don't think there is such an app. 
You either send message to yourself (Viber, Skype etc.) and your parents answers to you (but basically to themself at the same time) or they sign up to something (perhaps you could sign them up, without having them do it). 
I have an idea, but that's kinda "original" (aka stupid): use something like TeamViewer to take control of the pc with your S3 and just use a blocknote to write. 
Easy, fast and no one has to signup to nothing xD