Theoretically, could a ramjet-scramjet-rocket combination exist that operations like a rocket for taking off(below operations speeds from the ramjet or scramjet) or in space, and then switch to ramjet/scramjet mode when accelerating up to orbital velocities?
This is called a "combined cycle" engine and has been studied a lot, maybe flown a few times on experimental basis:
The RBCC, or rocket-based combined cycle propulsion system, was one of the two types of propulsion systems that may have been tested in the Boeing X-43 experimental aircraft. The RBCC, or strutjet as it is sometimes called, is a combination propulsion system that consists of a ramjet, scramjet, and ducted rocket, where all three systems use a shared flow path.
Source: Wikipedia article and its sources