Plugins in the mon itorin chain create a delay in the monitoring chain. Question is how big is the Plugin delay. I don´t have waves L3, so I can´t tell you.
I think it is expalined quite well in the manual → monitoring via Cubase / direct monitoring.
In short: a plugin needs time to calculate and Cubase has full Plugin delay compensation, therefore all signals get delayed the amount of the “slowest” plugin, so all signals are delayed the same and are therefore in sync overall.