  The nature of the skyrmion-crystal (SkX) formation and various multiple-$q$
phases encompassing the SkX phase are investigated by extensive Monte Carlo
simulations on the frustrated $J_1$-$J_3$ triangular-lattice Heisenberg model
with the weak easy-axis magnetic anisotropy. Phase diagram in the temperature
$T$ vs. magnetic-field $H$ plane are constructed, leading to a rich variety of
multiple-$q$ phases. The anisotropy stabilizes the SkX state down to $T=0$ at
intermediate fields, while in the lower-field range the SkX state becomes only
metastable, and new multiple-$q$ states with a broken $C_3$ symmetry are
instead stabilized. Implications to experiments are discussed.
