Park hopper has to be applied to an entire ticket regardless if the number of days on that ticket, I believe.
But you can buy individual admission days and then apply park hopping to any one of those. I don't think it will save much money and may even cost more.
Adding Park Hopper adds it to all the days of your ticket. Not just one. The good news is that the extra cost for a 3 day ticket is the same as a 1 day ticket. (around $50)
Park hopper has to be applied to an entire ticket regardless if the number of days on that ticket, I believe.
But you can buy individual admission days and then apply park hopping to any one of those. I don't think it will save much money and may even cost more.
The nice thing about the hopper, even though your intentions may be to stay at one park for the day is that if it gets too crowded or you just want to go to Dinner at EPCOT after spending the day at MK or go see Fireworks etc you can do that. We only buy hoppers to have the flexibility. If you go to DisneyWorld.com and look at tickets you will see the price per day really drops as you add days. While you mention a total of 3 days adding a 4th only adds $30 per ticket (not including hopper cost).
OP, a word of warning, in case you were considering this - you can't buy a 4 day ticket for a 3 day trip with the idea of using 2 different admissions on the same day (as in DAK and HS). You would have to buy a hopper.
Some of the best advice I ever got is that you can add hopper later. Since then, we have gotten regular tix with the intent of adding hoppers if and when we need/want them. So far, we have not added them.
Interesting article in the New York Times - guy got a one-day park hopper for $155 at a hotel and then hit all 4 parks to do the coasters. He was able to link his ticket to FP+ to get late FP for MK but used single rider lines and good timing to hit the other coasters.