From perusing the top-voted questions on each tag, here's my analysis:
islamic-rules and islamic-ruling seem to be exactly the same thing (I wouldn't be surprised if -ruling actually originated as a mistyped -rules) which looks to be a general tag for "What does Islam say about…" or "Is such-and-such permitted in Islam…" style questions.
islamic-law doesn't appear to be much different, but it does seem to tend towards more government-based questions (as does, to further complicate things, sharia in pretty much the same manner).
While I can sorta recognize the value of keeping "rules for Muslims" and "rules for Governments" separate, I don't really see why that needs its own tag over simply double-tagging with islamic-rules (or whatever) and islamic-government.
And even though islamic-law tends toward government-related questions, its use is still very inconsistent. Many of the questions under the tag are clearly more in-line with the islamic-rules category above.
I really don't see the value in maintaining separate tags in this case. Too open to confusion, and too little actual payoff.
So unless someone is able to present a compelling argument toward keeping them separate (and given that this meta question has been up for two weeks and received zero love, I'm not holding my breath) I propose simply merging-and-synonyming all three of them to sharia.
And then we can go about figuring out how exactly this whole conflation should even be being used: Even when we all agree that these tags are actually the same thing, I'm still not convinced anyone really knows what that "same thing" is supposed to be. It mostly just looks like a catch-all for "I'm asking something that probably involves a rule somehow".
But that's a whole other discussion.