  Interaction of three-dimensional magnetic fields, turbulence, and
self-gravity in the molecular cloud is crucial in understanding star formation
but has not been addressed so far. In this work, we target the low-mass
star-forming region L1688 and use the spectral emissions of $^{12}$CO,
$^{13}$CO, C$^{18}$O, and H I, as well as polarized dust emissions. To obtain
the 3D direction of the magnetic field, we employ the novel polarization
fraction analysis. In combining with the plane-of-the-sky (POS) magnetic field
strength derived from the Davis-Chandrasekhar-Fermi (DCF) method and the new
Differential Measure Analysis (DMA) technique, we present the first measurement
of L1688's three-dimensional magnetic field, including its orientation and
strength. We find that L1688's magnetic field has two statistically different
inclination angles. The low-intensity tail has an inclination angle
$\approx55^\circ$ on average, while that of the central dense clump is
$\approx30^\circ$. We find the global mean value of total magnetic field
strength is $B_{\rm tot}\approx135$ uG from DCF and $B_{\rm tot}\approx75$ uG
from DMA. We use the velocity gradient technique (VGT) to separate the magnetic
fields' POS orientation associated with L1688 and its foreground/background.
The magnetic fields' orientations are statistically coherent. The probability
density function of H$_2$ column density and VGT reveal that L1688 is
potentially undergoing gravitational contraction at large scale $\approx1.0$ pc
and gravitational collapse at small scale $\approx0.2$ pc. The gravitational
contraction mainly along the magnetic field results in an approximate power-law
relation $B_{\rm tot}\propto n_{\rm H}^{1/2}$ when volume density $n_{\rm H}$
is less than approximately $6.0\times10^3$ cm$^{-3}$.
