From the American English perspective, all three are grammatically correct. The difference is primarily on where the emphasis is being placed, and by extension, the possible additional meanings that might be inferred, without any further context. However, remember that context is everything!
The first, "Glad you have it resolved" emphasizes that the speaker ("I") is glad, and that it's because the problem is resolved. It's somewhat open as to how it got resolved. It could be because "you" solved it, or because someone or something else solved it for you.
The second, "Glad you have resolved it" emphasizes the participation of you in getting it resolved, whether by direct action or indirectly by having someone or something else take care of (resolve) it for you.
However, the difference between those first two is very subtle, and open to disagreement among native English speakers. Both are 100% grammatically correct for all meanings that either might have. Both can mean that you solved the problem, or that you had someone or something else solve the problem, or that the problem just resolved itself without any action (e.g., the drought ended because it rained).
The third, "Glad you got it resolved" is effectively the same exact meaning as the first, but using a more informal (and more common in spoken word) verb "got" instead of "have".
The second sentence does sound unnatural. "Tell" needs an object in this case.
The first and third sentences both work, and mean almost the same thing. The third sentence focuses on telling you something. Out of context, it sounds more like a direct warning -- "Hey, pay attention to this, car mechanics are trouble!" The first sentence sounds more like a remark or opinion -- "Speaking of car mechanics, I've met a lot of dishonest ones." It might be agreeing or disagreeing with another sentence. These are only small differences, though.
Also, you might want to put a "the" before "car mechanics". It makes the sentence sound smoother since you're talking about specific car mechanics (the ones you come across).
Best Answer
Any use of the pronoun "it" requires an appreciation of its definition:
If you are asking someone to tell you something then it is fair to say you both must know what you are referring to, so let's take it as a given that you are asking to be told something "previously mentioned". Neither of your requests would make sense if the other party had no clue what you were talking about.
The thing to consider is if you are asking to hear about something "clearly identified". "It" refers to one singular thing. For example, you could ask someone to tell you a story, or a joke. You can count stories and jokes, and you can refer to one as "it", eg:
It would not be correct to say "tell it to me" when referring to something plural, or "collected", for example:
You can't refer to "details" as "it", because it is a plural noun, ie there is more than one detail. You could say "tell them to me", but that may not be idiomatic as some plural nouns are used as "collective" terms and you do not expect to count the items within. "Details" usually means "all the information" about something and is not necessarily something you bullet-point and count, so in this instance "tell me" is the most idiomatic response.