Unlike mammals and birds that use most of the food they eat to keep warm, most fishes take their body temperature from the surrounding water. This means any food they eat is used for growth and movement. This allows fish to go for extended periods of time without eating and not suffer any major issues.
I have left fish for 2 months at a time and they didn't get fed during that time. The tanks were well planted before I left and I increased the lighting times to encourage algae. When I got back the plants and algae were gone and the fish were hungry, but they didn't die.
So 3 days, even 1-2 weeks is not an issue for healthy adult fish. It's even less of an issue if the tank has lots of plants and algae in because the fish can graze on these.
The only fish that have an issue when there is a lack of food is sick fish or baby fish. With baby fish, having loads of plants in the tank and encouraging green water will keep them alive for weeks or even months without supplemental feeding.
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As for holiday feeding blocks, they are normally calcium based with layers of food in them. As the calcium dissolves, the food is released.
If the pH of the water is above 7.0, they don't dissolve, or they take a really long time to dissolve.
If the pH is below 7.0, they can dissolve completely in a day or two and pollute the water with uneaten food.
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Some things you can do before going on holiday to increase the fish's survival rates.
Feed the fish more often (3-5 times a day) for a few weeks before you go on holiday. This allows them to build up some fat reserves and they can live off that when you are away.
Do more frequent water changes and gravel cleans (every couple of days) when feeding more often. This cleans the gunk out of the tank and provides a cleaner habitat for the fish while you're away.
Do a water change a couple of days before you go.
Make sure any new water is free of chlorine/ chloramine before it's added to the tank,
Clean the filter a few days before you go so it doesn't block up while you're away. Don't change filter media, just wash the old media in a bucket of tank water and re-use that media.
Add a heap of live aquatic plants 3 or 4 weeks before you go and increase lighting times a few extra hours per day. The light will encourage plant and algae growth and the fish can pick at the algae while you're away.
Don't add anything new to the tank in the 2 weeks before you go because you can introduce diseases.
Feed the fish before you walk out the door and have a nice vacation.
