This question relates to this topic:
Zorin OS 16 Ultimate is still in the making and has not yet been released. However, I agree with you that in choosing Ultimate this close to the anticipated release of Zorin OS 16 really should go toward Zorin OS 16 Ultimate. This is because you chose to help support development, not necessarily because you specifically wanted / needed Zorin OS 15.3.
Zorin OS 16 Core is anticipated to be Fully Released (Currently, many of us are using Beta) within the next couple of weeks or so.
You can send a PM to @azorin or @zorink right here on this forum, or use the "Contact US" form that came with Zorin OS 15 Ultimate to ask the developers for assistance with your dilemma.