Edible insects are advocated as sustainable and healthy food and feed. However, commercially produced insects are often low in n-3 fatty acids and have suboptimal n-6/n-3 ratios. A certain amount and proportion of these FAs is required to optimize human health. Flaxseed oil consists primarily (57%) out of alpha-linolenic acid. An experiment was conducted to quantify the effect of flaxseed oil provision on fatty acid composition and to determine the quantity needed to attain a beneficial n-6/n-3 ratio. Three species were used in the experiment: house crickets (Acheta domesticus [L.]), lesser mealworms (Alphitobius diaperinus [Pfanzer]) and black soldier flies (Hermetia illucens [L.]). These were provided with either a control diet or a diet enriched with 1%, 2%, or 4% flaxseed oil during their larval/nymphal stage. Fatty acid profiles of diets and insects were determined via GC-MS. The three species had distinct fatty acid profiles on all four diets, but responded similarly to flaxseed oil addition. For each percent added to the diet, the alpha-linolenic acid content of the insects increased by 2.3%–2.7%. Four percent addition increased the n-3 fatty acid content 10–20 fold in the three species and thereby strongly decreased n-6/n-3 ratios from 18–36 to 0.8–2.4. A ratio below 5 is considered optimal for human health and was achieved by 2% flaxseed oil inclusion for house crickets and lesser mealworms, and at 1% inclusion for black soldier flies. Adding a source of n-3 fatty acids to insect diets can thus improve the nutritional quality of insects.