Heart Rate Training why no Zone 3 (Page 2)

I do 2-3 5 mile recovery work outs a week in my Z1 Heart Rate Zone

I do 1-2 medium 10 mile runs a week in the Z2 Heart Rate Zone

The weeks I do just one medium run I will do either a Hill work out, track work out, or tempo run for 6-7 miles.  My hill work outs have 100-150m all out sprints in my Z5+ zone, my track work outs are 400m repeats in my Z4 zone, and my tempo runs are 8-20 minute intervals in my Z3 zone. 

I do a 12-15 mile long run in my Z2 zone each week.

The bike training plan is two medium rides a week and a long ride and has a similar range of heart rate zone.  About 80% of what I do is in the Z1-Z2 zone.  The other stuff is basically just breaking up the pace.  I haven't read much on Heart rate training.  I came to Triathlon in 2015 from a 30 year competitive running background.  As a runner I had 7-8 coaches and not a one of them every had me do a lick of heart rate training.  I am convinced that Heart Rate training must have come to the triathlon culture from cyclists.  As a runner I would do speed work outs on the track every Tuesday and everything else was Z1 running unless we had a race on the weekend then it would be Z5+.