@Josh *********
then you can't do it. Marriage non-O usually gets a 30 day under consideration stamp and they might want to do a home visit. 3 weeks is not enough time. You'll need to do it when you return or you can get it from Thai embassy in your home country and then apply for a re-entry permit before you leave for work, then apply for your 1 year extension when you return from your job.
If you already have the 400,000 in your Thai bank account shouldn't be any issues in applying for your extension.