Everybody need a COE since August 2020. Doesn't matter if you're coming with or without visa, you'll need a COE for people at Bangkok airport to prepare for your arrival.
If your end of allowed stay is before or on 30 September, you can ask for an extension.
If your end of allowed stay is after 30 September (that will obviously be the case of people entering the kingdom from 3 July onward) no extension request accepted. Period.
Every STV holder who is permitted to stay in Thailand any date up to 30 September can request an extension by going to the immigration office before or on 30 September.
People allowed to stay from 1st October onward, will not be eligible to request such an extension.
Therefore, every STV holder who enter Thailand from 3 July onward, will not be eligible for an extension of stay, because they will be allowed to stay at least up to 1 October (90 days) which is after 30 September.
I know perfectly that when you request for an extension you give them all the detail about your address, they put the data in their computer and the system assume you reported yourself and reset the 90 days to day 1. So please people, stop saying that immigration rules are nonsense when you are those stubborn who doesn't want to understand.