  We investigate the nonlinear algebra $W_3$ generated by the 9 functionally
independent permutation-symmetric operators in the three-particle rational
quantum Calogero model. Decoupling the center of mass, we pass to a smaller
algebra $W'_3$ generated by 7 operators, which fall into a spin-$1$ and a
spin-$\frac32$ representation of the conformal $sl(2)$ subalgebra. The
commutators of the spin-$\frac32$ generators with each other are quadratic in
the spin-$1$ generators, with a central term depending on the Calogero
coupling. One expects this algebra to feature three Casimir operators, and we
construct the lowest one explicitly in terms of Weyl-ordered products of the 7
generators. It is a polynomial of degree 6 in these generators, with
coefficients being up to quartic in $\hbar$ and quadratic polynomials in the
Calogero coupling $\hbar^2g(g{-}1)$. Putting back the center of mass, our
Casimir operator for $W_3$ is a degree-9 polynomial in the 9 generators. The
computations require the evaluation of nested Weyl orderings. The classical and
free-particle limits are also given. Our scheme can be extended to any finite
number $N$ of Calogero particles and the corresponding nonlinear algebras $W_N$
and $W'_N$.
