In Studio One there is a side chain button on side chainable plugs but in Sonar X3b no side chain button. I though X3b fix the side chain issue.
 
 
Really liking X3

There's no button. You route the output or create a send from the channel you want to use for the side chain signal, and as destination you pick the sidechain input of your plug. This didn't really change in X3 in the sense that the Sonitus Gate/Comp has had this since the beginning. X3 added VST3 support though which makes sidechaining available for any VST3 plug that supports it. E.g. In my Waves bundle Renaissance Compressor and Renaissance Channel. The way to do it is still the same.

I thought all VST plugs would show the sidechain button. OK so nothing has changed in the way you sidechain in Sonar. Thanks.

For most plugs it makes no sense to have a sidechain input. So yep, nothing has changed except VST3 support including sidechaining for plugs that support it.