Is it too late to apply for a marriage visa extension 20 days before the expiry date in Thailand?

What’s expiring your 90 day Non O or are you already on an extension ?

I believe you're confusing the process of applying for an extensions with the process of applying for in country 90 day Non-Imm visa <- which requires you have so many days left on your current stamp (most offices are minimum of 15 days, some offices (Chiang Mai) are 21.

At most immigration offices you can apply for an extension when you have 30 days or less left on your current stamp (Some Bangkok, Chiang Mai, Samut Prakan, etc allow you to do it when you have 45 days or less left)

You don't lose any time applying for a new extension (for the same reason) because the extension is added to the expiration date of your current stamp

So you can apply for a new yearly extension based on marriage from 30 days before right on down until the day your current stamp runs out..

20 days before is a prime time to renew, the 3 weeks you are thinking about is the under consideration period, don't worry about that, you will still have a valid extension.