Will I notice a pressure drop going from 1/2" pex to 3/4" CPVC then to 1/2" CPVC?
It truly will not be as efficient as 2 separate lines feeding each fixture, or 3 separate lines for 3 fixtures.... You will only notice a pressure drop when both fixtures on the same branch are used at the same time. You will see the difference, it may not be drastic enough to need to gut what ever is in the way to get it run  everywhere. If I remember right, Maniblock only has 1/2" connectors?