Funnily enough I got a complete mirror delivered off eBay yesterday. For all you high rollers in the gen 3 GS, mine was 25 quid delivered I only really needed the glass too.
Now keeping in mind mine is second generation and yours is third, I popped in to see Dave at Lex-Tek but needn't have because replacement was very easy. There are 4 clips holding the glass to the backing. Swivel the glass so the mirror is pointing up and pull off the bottom two first and then the top two. Undo the connector and you're done. Fitting is the reverse.
Again, there's a few years difference between ours but it shouldn't be vastly different. Hopefully the above will be of help.