Thai embassy in the Netherlands made separate pages for the 3 different DTV options.
All 3 options have the financial requirement of 500k THB / €
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which seems logical to me. I see no reason at all why the third option should not require funds.
I think most easy is when she has own account, but maybe a common account with double the money also might work.
I read somewhere some embassies now also want proof that the funds are not there just for the moment of apply for visa
DTV3
Required Documents
1. Biodata page of Passport or Travel Document
2. Photograph taken within the last six months
3. Document indicating current location
4. Financial evidence: amount of no less than 500,000 THB, e.g. bank statements, sponsorship letter
5. DTV visa of the DTV holder
6. Proof of relationship to the DTV visa holder e.g. copy of marriage certificate/ birth certificate / certificate of adoption