This is correct. You must install the 2012 R2 SP1/2012 SP2 client agent first before it can be patched to CU3.
You can use multiple methods to do this but the easiest is to just enable client auto upgrade: https://blogs.technet.microsoft.com/configmgrteam/2015/08/04/automatically-updating-the-configuration-manager-client/
Jason | http://blog.configmgrftw.com | @jasonsandys