I have a marine color dosing pump that does mine everyday for me. It's perfect for when I'm out of town for a couple days. Best thing I could have done!!! I don't have to worry about dosing at all anymore. I set it up to dose Marco at a 1/2 dose 2 times a day morning and afternoon and micro in the morning. All before the lights come on.
You can also lessen your photoperiod and co2 by half for those 3 days. I've went on a week long vacation by doing this and plants were healthy except for being a bit leggy. Less light, less need for ferts. 3 days should not be that big of a deal unless you are running high light.