Am I right in thinking that is your normal 4 bolt standard?
Yep.
So buying a FSA BB30 chainset 175mm 104/64 reduced from more than £200 to £60.. only for the 3 rings to go on my Shimano SLX at some point..
Not a silly idea. Total compatibility right?
Maybe, you may have to file things down, either the spider or the rings. But yes, on paper they'll fit.
Shimano middle rings are offset compared to other brands, not sure which way tho, you might be ok with some shims. TBH £60 for just rings, you might be able to do better than that (those FSA ones will prob be crap anyway).
just looked on CRC, you can get Middleburn Hardcoat for £63! A FAR FAR better choice.
Or just buy a new SLX chainset and flog yours when the time comes. That's what I'd do.
+1 for middleburn.
or you could buy these...
[url= http://singletrackworld.com/forum/topic/deore-chainringsbrand-new ]clicky[/url]
Are Shimano middle rings offset?