Thanks all for the replies.
Yes, Zdas04, you're correct, that is what i used to know about it (tri-cock). But, the term came up at my work site when level gauge on site and transmitter to control room becomes inoperative on the electrostatic dehydrator. The only way to know the liquid level is by sampling by means of set of valves arranged in slopy manner (see attached for e.g.). And, when i searched on-line, i didn't really see any clear explanation to that.
@Mike, thanks for the link, i also came across it during my search on-line.