Three days overstay it’s not advisable, go to your local and nearest immigration and apply for a 30 days extension. You’ll be fined with 1500 baht anyway and also get a overstay stamp in your passport.
As you will be able to prove that you applied for your tourist visa while you were present and also left from Namibia, you’ll probably be able to do a border bounce and activate your tourist visa when you receive it also if you need/want more time as well.
No, the easiest thing is really to apply for a 90 days Non O online at home before you come if that’s an option. Then you can use income or funds, and you’ll do everything more easy for yourself and just can prepare for the one year extension of stay within 90 days if that’s what you plan to do.
You can apply for a 90 days Non O visa in country if you arrive on a visa exemption or tourist visa if you fulfil the requirements and have minimum 14-21 days left on your stamp.
You’re biggest issue is to open a bank account for banking the required funds, unless your embassy still offers to give you an affidavit letter regarding income.
Can add that I tried again today at Krungthai Bank in Pattaya with the same Non O visa and got the same answer; Yellow house book OR a work permit as well.
(Same branch who told me two years ago that I could open a second account only with a Non O visa as I showed up with an exemption. Definitely getting more strict now).
Personally I would say it’s more convenient to apply for the initial 90 days Non O visa before you come and apply for the one year extension of stay in Thailand.