Are chainrings specific to double or triple cranksets? I have a 2008 Specialized Allez with a triple crankset. The large and middle rings were getting worn so I bought replacements of identical tooth counts for them. However, after installing them I found it was impossible to shift onto the largest ring no matter how I adjusted the front derailleur because the space between the middle and largest rings was just too wide.

So I removed the rings and compared them. The new large ring was identical to the old one but the middle ring was not. The tabs where the bolt holes are drilled in the old ring were thinner than the new ring. The result was this made the middle ring stand about 2 mm farther from the large ring, just enough to make shifting upward impossible.

Is it standard for middle chainrings on a triple to be thinner or was that just unique to the crankset used on the bike?  (It was the stock, unnamed crankset installed by Specialized.)
Absolutely the middle chainring of a triple is different to the inner and outers.  


Tooth profile is different because the chain has to move either way from the middle
Ramps are different - your middle needs ramps on the inside in a triple.
Mount offsets are different too, which is why a triple front derailleur is different to a double.  The amount of cable pull is less for a triple compared to a double.


So if you can return the bad chainring, do so.  If not, and you're willing to modify it, you could try grinding away the tabs a little.   Don't bend them too much, alloy won't like being bent and you'll weaken the whole thing.

Probably safest to list it on ebay and buy the right part - your LBS would be best rather than buying parts on-line.  Plus if they sell you the wrong part, you can return it.