I just posted this on another topic but wanted to re-post it here because it is relevant to this discussion:
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To my knowledge there isn't a way to run a cycle on every 3rd day. The reason we didn't implement every N day on LawnCheck because most people want to know which days of the week their sprinklers will run - especially during mowing season.
You can more closely estimate an every-third-day schedule using two programs: set one program to run on a 3/4 schedule, for example Monday and Thursday, and then create another program to run once a week, for example on Saturday. You can scale back the irrigation times on the Saturday programs as needed. This will give you a 3/2/2 schedule
Also, in many climates a lawncare pro will recommend a 3/4 schedule instead of a 3/2/2 - with longer irrigation times - this gives the water a chance to soak in. In some very hot climate this might not work. LawnCheck gives you 5 independent programs per controller with monthly controller on each, so in hot climate you might want to add a few "summer-only" programs.
Jim