I have a tourist visa till 11th July. My plane leaves on 20th. I will go to immigration to get a 7 day extension. That will put me till July 18th. I don't want to overstay. Can I get another 3 day extension?

I also have to be in Pattaya and can do Pattaya Immigration or Bangkok Immigration. Which is better? Easier to get an extension? How long of a wait normally? Thanks.

If you have a tourist visa (got visa from consulate) you'll get 30 days extension.

If you arrived on a visa exempt stamp (given at airport) you'll get 7 days if you can show a ticket out of the country within 7 days.

No further extensions are given.

If you are on a visa exempt stamp you should go to nearest border and get another 15 day stamp.

As said a Tourist Visa can be extended for 30 days at Immigration.