  We present a scheme to generate shell-shaped droplet in a three-component
(1,2,3) ultracold Bose gas. Here binary mixtures (1,2) and (2,3) form quantum
droplets due to inter-species attractions, and the two droplets are mutually
immiscible due to strong 1-3 repulsion. Importantly, the shared component-2
serves as a glue to link the two droplets together as a globally self-bound
object. In this system, the outer droplet naturally develops a shell structure,
and its radius and width can be conveniently tuned through the size of core
droplet. Moreover, to reach an equilibrium with the shell, the core droplet
displays very different spin densities as compared to the vacuum case. These
results have been demonstrated in a realistic $^{23}$Na-$^{39}$K-$^{41}$K
mixture. Our scheme liberates the shell-shaped Bose gas from stringent
conditions with microgravity or fine-tuned traps, and can be readily
implemented in cold atoms laboratories on Earth. This paves the way for future
exploration of quantum droplets in curved space with non-trivial real-space
topologies.
