How to understand "喝茶"？
I start with 2. It's the literal meaning of 喝茶. 喝 = drink, 茶 = tea, so "have some tea" is a perfect translation of 喝茶.

To explain 1, you should think it as an interpersonal communication. In some cases, 喝茶 means "sit down and have some drink", usually after a guest visit. In this case, 茶 does not necessarily be 茶 and may be any drink. It's usually when people have something to talk or discuss that the host serves some drink first.

The 3rd one as recognized by 神秘德里克, is an internet slang. When one talks about something politically-sensitive (like the Tiananmen Square Massacre) in mainland China, local police would ask them to “来警察局喝点茶” (come to the police office and talk seriously), which then boils down to the above paragraph (serve drink before discussion).