Relative position of three sensors
This is not a trivial problem. Depending on what exactly your ultimate goal is and the resources at your disposal, there could be a number of different approaches.

IMUs (inertial measurement units) - sensor units with accelerators, gyroscopes, and sometimes magnetometers - will not be enough to accurately determine relative positions of wrist and shoes. They will give you a lot of information about motion but if you try to infer position from that information, the error will add up and you will get inevitable drift.

Normal GPS is not a good idea for something this small scale (as suggested in a question comment) since it just won't be accurate enough. Differential GPS is slightly better there but might be completely unsuitable for your application and you might find it isn't worth it. Similarly, most RF approaches will heavily fluctuate in accuracy simply based on how a person is standing or where there are walls. Those are all long range solutions.

Again, depending on your application (indoors? outdoors? large area?), your best bet is probably a computer vision approach. There's motion capture where you attach markers to the parts you want to track and put cameras at as many angles as possible such that as many as possible always have an a view that is not occluded of these markers. With those video streams and some maths, you can very accurately track the positions of the markers. This is done in the film industry for example to get motion data for CGI.

Otherwise you could always grab a Kinect and make do with the data that that gives you (which may still be better than attaching IMUs) and maybe even fuse that data with a different tracking method.