  Spin triplet superconductor UTe$_{2}$ is widely believed to host a
quasi-two-dimensional Fermi surface, revealed by first principal calculations,
photoemission and quantum oscillation measurements. An outstanding question
still remains as to the existence of a three-dimensional Fermi surface pocket,
which is crucial for our understanding of the exotic superconducting and
topological properties of UTe$_{2}$. This 3D Fermi surface pocket appears in
various theoretical models with different physics origins but has not been
detected experimentally. Here for the first time, we provide concrete evidence
for a relatively isotropic, small Fermi surface pocket of UTe$_{2}$ via quantum
oscillation measurements. In addition, we observed high frequency quantum
oscillations corresponding to electron-hole tunneling between adjacent electron
and hole pockets. The coexistence of 2D and 3D Fermi surface pockets, as well
as the breakdown orbits, provides a test bed for theoretical models and aid the
realization of a unified understanding of superconducting state of UTe$_{2}$
from the first-principles approach.
