This is an interesting question because it involves the use of next/back buttons. From a UX perspective, both are associated and relate to navigation and so should be placed adjacent to one another as not only is it logical to do so, it follows a design pattern anticipated by users.
The next button should be placed to the far right, as this tends to be an indicator of a content change/confirmation section- additionally think of it like turning the pages of a book, the main flow is to turn to the next page (users rarely go back in comparison), so the button should be placed on the 'page edge' e.g. the far right.
This means the back button should be adjacent to this on the left, because it relates to the next button, and the assumed position will be beside it to the left.
This simply leaves the cancel button, the remaining place being to the left of the back button, however as it isn't a relates navigation control, per se, you may want to separate it from the back./next buttons with some additional spacing to make its purpose clear and reduce the chance of erroneous button clicks.