“How to prepare a vegetable garden wood dimensions (80 cm x 3) to plant tomatoes and zucchini in good conditions (substrate, organisation…)?”
for you to cultivate in the open ground or in-ground, the quality of the culture substrate is an essential basis. For crops raised (as you want it to do), the best way is to bring in topsoil enriched with organic matter at the surface, in the first 20 cm. Every year, as in any garden, you incorporate the compost to replenish the organic matter and thus humus and the fertility of the substrate.
Regarding the organisation of the cultures, the needs of the vegetables are the same regardless of the setting of the garden: exhibition (sunshine, heat), area of development, height and harmonization between cultures, principles of associations to prevent the spread of disease.
These points to be respected are all the more important that the cultures are carried out on small surfaces (such as 80 cm to the side of which you speak).