They're basically the same, except, THING could be positive or negative, whereas the other two are definitely negative.

They are all different. Only "trouble" is negative. "Thing" is the broadest possible category. It is completely neutral. Everything is a thing. When someone says "The thing is..." it is the same as saying "The point I want to emphasize is..." An "issue" is a topic that merits consideration. The word has no negative or positive connotation. It is close in meaning to "question". It can be used in situations where the question involves controversy or some sort of problem, but the word itself does not imply that. For example, scientists studying the diet of fruit flies could investigate the issue of whether they prefer eating sweet things or sour things. There's nothing positive or negative about that; it's just a question they want to consider.

These phrases, "The thing is...", "The trouble is...", and "The issue is...", are similar in that they are all introductory phrases commonly used in spoken or written communication to introduce a point or to provide an explanation. They are often followed by a statement that elaborates on the topic being discussed. While they serve the same general purpose of introducing a topic, they may slightly differ in terms of the tone or emphasis they convey: "The thing is..." suggests that the speaker is about to provide a specific point or piece of information that is crucial to the discussion. "The trouble is..." implies that there is a problem or difficulty that needs to be addressed or explained. "The issue is..." indicates that there is a matter or topic of concern that is going to be discussed or analyzed. In essence, while they share the common function of introducing a topic, they may convey slightly different nuances based on the context in which they are used.

Same

No