Q.Plant food 3:1:6
Can I sprinkle plant food in the garden and then just water? Or do I have to dissolve in water first?

It is always best to follow the application directions that the fertilizer packaging has.
Fertilizers may be granular or liquid or come in spikes.
Some are meant to be sprinkled in the soil and worked in or diluted in water.
Misused fertilizers can severely damage plants and even kill them.
To much Nitrogen can cause lack of flowering or fruit and vegetable formation.
Here are some links with more information.
https://www.gardeningknowhow.com/garden-how-to/soil-fertilizers/best-times-to-apply-fertilizer.htm
https://www.gardeningknowhow.com/garden-how-to/soil-fertilizers/giving-plants-a-boost-with-conventional-fertilizer.htm
https://www.gardeningknowhow.com/edible/vegetables/vgen/fertilizer-options-for-your-vegetable-garden.htm