Specifically hydroxylates an Asp or Asn residue in certain epidermal growth factor-like (EGF) domains of a number of proteins. 2-oxoglutarate + L-aspartyl-[protein] + O2 = 3-hydroxy-L-aspartyl-[protein] + CO2 + succinate Monomer. Might be processed to the 56 kDa (AA 289-754) or 52 kDa (AA 311-754) forms in the lumen of the endoplasmic reticulum. Belongs to the aspartyl/asparaginyl beta-hydroxylase family.