Is it obligatory to fulfill both oaths (one rak’ah and three rak’ahs) after having sworn to perform one rak’ah and then three rak’ahs every time masturbation occurs, or is it sufficient to fulfill the greater oath (three rak’ahs)?

The prohibition of masturbation and some advice on how to stop it have been previously explained. If you swear to abstain from it and then break your oath, you must pray a complete rak'ah with its two prostrations – which is the view of the Shafi'is – or perform an expiation for the oath (kaffarat yamin). If, after that, you swear to pray three rak'ahs, you are obligated to fulfill that every time you commit the act, or perform an expiation for the oath. The number of rak'ahs that are binding depends on the intention of the one who swore the oath at the time of swearing it. If the intention cannot be changed, then you must either do what you swore to do or an expiation for the oath becomes incumbent upon you.