Gary Collins -- Jailbound

Gary Collins has just pled no contest to driving under the influence of alcohol -- his third one -- and he's been sentenced to 120 days in the pokey ... but there's a catch.

Gary can apply for house arrest with an electronic monitoring device -- though there's no guarantee that'll happen.

Collins was busted at the end of January for driving a motor home drunk around Santa Barbara -- so drunk, in fact, his BAC was .29...more than 3 times the legal limit.

Collins will also have his vehicle impounded for 90 days, and his license will be suspended for 3 years.