I have *not* yet tested I3 w/ the WHMCS 5.3 RC versions... I have tested JWHMCS and my other modules against one of the earlier beta releases and found no issues in the template rendering.
I will be working on 5.3 next week, but won't be pressing too hard on it unless they do a full release. The last time they did a major release (5.2) there were incredible changes between the beta product they said was 5.2 and the final product. We spent a week tracking down and accommodating the changes only to have all the changes reverted in the subsequent 5.2.1 release they did. I'm hesitant to invest the time in ensuring our products work with their beta products at this time.