Well, I don't have that much experience with HAI's rules but using a flag or phantom output or something like that would be the logical way to do it. 18 hour limitations on those seems to be very limiting. Unless there is a very good reason I think HAI should put that on their list of enhancements for the next firmware release. Either extend hours to unlimited or add a Days count in addition. The Elk M1 lets you do days so I think this is really an area where HAI should improve.
All that said, the following strategy should work. There also may be a much better way I am not familiar with. This is theory and not tested but I think it will do as desired. Unfortunately it ties up 5 flags and blocks.
Set up 5 flags, my example uses Irrigation, 18-1, 18-2, 18-3 and 18-4. Since you can't just set the 1 flag for the 3 days... Irrigation is the master flag that indicates that you are in the 72 hour (3 day) disable window due to pressing say Button1. 18-1 thru 18-4 represent 4 18 hour (max flag time) time blocks to equal 3 days. So, when Button1 is pressed turn on the master flag Irrigation and the first 18 hour block flag. Then, when each 18 hour block is up (flag off), as long as the master flag is still on, then set the next 18 hour flag. When the 4th flag turns off (end of 72 hours) then turn the master flag off.
For your irrigation you would simply check and if Irrigation Flag is on then skip your watering. You could also set another button to turn off the irrigation flag and all 18- flags to cancel your 3 day skip command.
Here is what it would look like using the above flags and Button1 to set it.
The other option would be to set a counter, like maybe a user setting to 3 and decrement each day at midnight or something, but I don't see a way to increment or decrement a user setting/number. This is also probably something HAI should look at for improvement. In the M1 to can modify counters in rules and its very handy.