The three things are doing pretty different jobs, and so should be pretty different in content.
The abstract is an advert for your paper. Someone might read a list of titles and abstracts (for example on arXiv) and decide on that basis whether it's worth their while downloading and reading the rest of the paper. The abstract has to be attractive, but not fraudulent ('truth in advertising'!). When you read an abstract, you're trying to find out: (i) is this paper in my area?, (ii) is it asking a question I care about? (iii) does the conclusion sound interesting? You don't have to summarise the whole paper, but you do have to answer those questions.
The introduction gives your reader a map of the paper. By the end of the introduction, your reader should have a pretty good idea of what they're in for, and where they're going to end up. They should know what sections they're probably going to skip, and which ones they look forward to disagreeing with. A paper is not a detective novel: you don't need cliffhangers or mysteries.
After that, the conclusion doesn't have a lot of work to do (at least in my experience). It might be the last thing the reader reads of your paper (that is, unless they care enough about its contents that they'll re-read it), so this is an opportunity for you to frame their memory of it. "We have shown that ..." is the sort of thing you'd find here.
- Remember the reader is a human being, just like you – talk to them, as you'd like to be talked to (far far too many people bizarrely forget this).
- Make it easy for them to read the paper, and easy to agree with you (that's why the introduction has a map of the paper, to ease the reader's way through your golden prose).
- Your reader is asking themself "why am I reading this paper?" Make sure they have a good answer to that question by the end of the introduction. If you get a colleague to read your draft, get them to answer that question at that point. If they don't give the answer you want them to, it's your fault; so edit.
There will probably be some overlap in text in the three parts. That's OK, but it probably shouldn't be cut-and-paste.