Anybody know how to include the tracer particles (PartType3) in the cutout.HDF5 files?
Unfortunately this isn't possible - because the tracers are not "group ordered" like the other particle types. Typically to locate the tracers corresponding to a set of gas/star/BH parents, a search over the full snapshot would be done. If following them through time, this will anyways become necessary at higher redshift, because they will in general no longer reside in the main progenitor branch (and/or full merger tree) of the target halo.
You could, in theory, download "full snapshots" with only the tracers following "Example 11" of the API Cookbook, and make these type of searches.
Alternatively, the full snapshots are now available on ODIN under /virgo/simulations/Illustris/.