Is it safe to apply zinc, iron and boron together, or will they react and waste each other?

You can correct several micronutrients in the same season, but how you combine them needs care. For soil application at sowing, zinc sulphate and borax can both be applied, just keep boron in a small, accurately measured amount because its toxic threshold is close to its useful dose, and mix it very evenly through the soil so no plant gets a concentrated patch. Iron applied to soil is often a poor investment on high pH soils because it gets locked up, so iron is usually better corrected by foliar spray. For foliar sprays, be cautious about tank-mixing many micronutrients at once: some combinations and some carriers can react or raise the concentration enough to scorch leaves, and certain ready micronutrient mixes are formulated to avoid this while loose mixing at the farm may not be. A safe practice is to follow your Soil Health Card recommendation, apply zinc and boron to soil in their separate measured doses at sowing, and use foliar iron or a balanced micronutrient spray only for the nutrient a test confirms is still short. Please confirm each deficiency by test before applying, follow your local KVK on which products can be combined and at what concentration, and do not over-apply, boron above all.