As you may or may not know, Arma 3 is now available in 64-bit version as well. This means that all the 32-bit extensions you have, will stop working with 64-bit Arma, and you will need to recompile them for 64-bit platform. The good news is that it would probably be pretty straightforward process. To test, I recompiled debug_console extension into 64-bit, and this was hassle-free experience.
So, 64-bit Arma will look for extension with the name ending _x64. To remember this, look at the arma3.exe and then 64-bit version of it arma3_x64.exe. This is basically what you should have, your normal 32-bit dll, say myextension.dll and 64-bit version of it, myextension_x64.dll if you want it to work with any Arma 3.
Another good news is that, in scripts you do not need to change the name of extension you call with callExtension into _x64 version. The game will automatically append _x64 and look for that file. So with my debug_console, the calling of it doesnt change on 64-bit version, will still be:
"debug_console" callExtension "12345";
as long as there is _x64 version of extension present too. If there isn’t, absolutely nothing would happen. So here you go, to read more about changes, have a look at the OPREP written by Richard Biely who made 64-bit happen.