What Marketo support can do is merge 2 fields that are mapped with a CRM lead and a CRM contact or with a CRM lead and a CRM account.
I am not sure they can merge fields that are marketo only. And even though, in this case, you would better copy all the value you want to keep from fields B & C into field A with smart campaigns using "change data value" flow steps and tokens as new values, then hide the B & C fields.
I believe that support should be able to do this as a two step process. If not you may need to export these fields and merge them in excel and then re import them