Custom System Messages is finally ready for ExpressionEngine 3, but with one draw back. You can't use it until EE 3.2 is released.
CSM 3.0 is nearly identical to the older EE 2 version, however, due to some architectural changes in EE 3 the System Offline template can not be changed.
There is no upgrade fee for CSM 3.0 and it is available immediately as a download in your account. The EE2 and EE3 versions of CSM are packaged in the same zip file. In a few months the EE2 version will be discontinued and no longer included in the download file.