The Cat3k product line does not have support for Flexible NetFlow (FNF) or Traditional NetFlow (TNF) on it's own.
Performance Monitor uses software based FNF on the cat3k and allows measurement of a filtered portion of the traffic going through the platform. The forwarding of the traffic continues to be done via hardware, and only the monitoring is done in software. So while a portion of traffic can be measured (and with performance metrics like packet loss, and jitter), this is a limited portion. The configuration of performance monitor is slightly different than FNF.
With the introduction of the Catalyst C3K-X service module, it is possible to do FNF on a cat3k at a hardware level performance. It is currently not possible to hardware accelerate the performance monitor function by the use of the C3K-X service module. The traffic between the acccess ports (not using the uplink ports) will not be metered. The configuration of FNF with C3K-X is the usual FNF configuration.