Can I get a 30-day visa exemption on my third entry in Thailand right before the year ends?

Assumption: You're from a visa exempt country (see 120-2-9.pdf).

1. You are entitled to two visa exempts by land each calendar year. Each entry is good for 30 days, which can be extended by another 30 days.

2. You are entitled to "unlimited" (as defined by the immigration officer on duty) by air. Each entry is good for 30 days, which can be extended by another 30 days.

3. There is no "180 day" limit, or a maximum of X entries by air. HOWEVER, the immigration officer at the airport will prevent you from entering if s/he thinks you're abusing the system and living here on visa exempts.

okay, you're mixing up entries. There is NO official rule on the number of 30 day visa exempt entries you can get by air. Although you're gonna get pulled aside, questioned and possibly denied entry after 3 or 4 of them especially if they're back-2-back.

There is a limit of TWO 30 day visa exempt entries by LAND in a calendar year (Jan 1 - Dec 31).

IF your entry history is correct and you have only entered without a visa by land once for a 30 day stamp you can do one more this year, THEN starting on Jan 1st you would be allowed two more 30 day visa exempt entries by land again for the year.

You should be able to do one more by land this year

I depends on the country of origin of your passport. Without knowing this all kind of assumptions could be wrong or correct