  We consider the statistical mechanics of a class of models involving
close-packed loops with fugacity $n$ on three-dimensional lattices. The models
exhibit phases of two types as a coupling constant is varied: in one, all loops
are finite, and in the other, some loops are infinitely extended. We show that
the loop models are discretisations of $CP^{n-1}$ $\sigma$ models. The finite
and infinite loop phases represent, respectively, disordered and ordered phases
of the $\sigma$ model, and we discuss the relationship between loop properties
and $\sigma$ model correlators. On large scales, loops are Brownian in an
ordered phase and have a non-trivial fractal dimension at a critical point. We
simulate the models, finding continuous transitions between the two phases for
$n=1,2,3$ and first order transitions for $n\geq 4$. We also give a
renormalisation group treatment of the $CP^{n-1}$ model that shows how a
continuous transition can survive for values of $n$ larger than (but close to)
two, despite the presence of a cubic invariant in the Landau-Ginzburg
description. The results we obtain are of broader relevance to a variety of
problems, including SU(n) quantum magnets in (2+1) dimensions, Anderson
localisation in symmetry class C, and the statistics of random curves in three
dimensions.
