For swim training, I've got a finger-mounted lap counter with a stopwatch feature. One button, one screen. Helps me make sense of training in a 33-metre pool with a pace clock only on one side (and bad eyes). It does the job, but I'd like something with more data though not at the expense of simplicity.
Cycling: Head unit, between the arms (and in front of stem, on the road bike) - that's where I'm staring anyway. I like my Joule GPS - very legible. Crucial variables (always on-screen) are 3sec power, cadence and HR. For racing I only have those. In training I like to add accumulated TSS, elapsed lap time, lap distance and others to track the session.
For running, wrist-mounted. Tried a HUD, disliked it.
What I'd really want is a stealthy recording-only unit. GPS and ANT+/BT, preferably on my back for decent swim data. I often race with a bulky triathlon wrist watch to record data for later viewing, but swim and transition times aren't useful
during the race. Having one unit record everything without interfering or creating drag would be great. Add a head unit for cycling and a wrist watch for the run, and perfect. Could even be a "slave" watch/headunit then (think Wahoo RFLKT) - reads from the recorder.
ZONE3 - We Last Longer