The main omega-3 fat in walnuts, flaxseed and chia is alpha-linolenic acid (ALA). The long-chain fats EPA and DHA are abundant in oily fish and in some algal oils. The body can convert ALA only to a limited extent.
A vegetarian or vegan pattern can still include ALA-rich foods most days. People who want EPA and DHA without fish usually look at algal oil products and should treat labels as the source of truth.
This article does not set a supplement dose. It flags a nutrient that is easy to miss if the diary only counts protein and iron.
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Sources
Fats and fatty acids — Nutrient Reference Values (NHMRC) https://www.eatforhealth.gov.au/nutrient-reference-values/nutrients/fats-total-fat-fatty-acids
Omega-3 fatty acids — NIH Office of Dietary Supplements https://ods.od.nih.gov/factsheets/Omega3FattyAcids-HealthProfessional/
Fats, oils and heart health — Heart Foundation https://www.heartfoundation.org.au/healthy-living/healthy-eating/fats-oils-and-heart-health