  We use large-scale Monte-Carlo simulations to study thermal fluctuations in
chiral $p$-wave superconductors in an applied magnetic field in three
dimensions. We consider the thermal stability of previously predicted unusual
double-quanta flux-line lattice ground states in such superconductors. In
previous works it was shown that, neglecting thermal fluctuations, a chiral
$p$-wave superconductor forms an hexagonal lattice of doubly-quantized
vortices, except extremely close to the vicinity of $H_{c2}$ where
double-quanta vortices split apart. We find dissociation of double-quanta
vortices driven by thermal fluctuations. However, our calculations also show
that the previous predictions of hexagonal doubly-quantized vortices, where
thermal fluctuations were ignored, are very robust in the considered model.
