Magnetars are regarded as the most magnetized neutron stars in the Universe. Aiming to unveil what kinds of stars and supernovae can create magnetars, we have performed a state-of-the-art spatially resolved spectroscopic X-ray study of the supernova remnants (SNRs) Kes 73, RCW 103, and N49, which host magnetars 1E 1841−045, 1E 161348−5055, and SGR 0526−66, respectively. The three SNRs are O- and Ne-enhanced and are evolving in the interstellar medium with densities of > 1 − 2 cm⁻³. The metal composition and dense environment indicate that the progenitor stars are not very massive. The progenitor masses of the three magnetars are constrained to be < 20 M_⊙ (11–15 M_⊙ for Kes 73, ≲13 M_⊙ for RCW 103, and ∼13 − 17 M_⊙ for N49). Our study suggests that magnetars are not necessarily made from very massive stars, but originate from stars that span a large mass range. The explosion energies of the three SNRs range from 10⁵⁰ erg to ∼2 × 10⁵¹ erg, further refuting that the SNRs are energized by rapidly rotating (millisecond) pulsars. We report that RCW 103 is produced by a weak supernova explosion with significant fallback, as such an explosion explains the low explosion energy (∼10⁵⁰ erg), small observed metal masses (M_O ∼ 4 × 10⁻² M_⊙ and M_Ne ∼ 6 × 10⁻³ M_⊙), and sub-solar abundances of heavier elements such as Si and S. Our study supports the fossil field origin as an important channel to produce magnetars, given the normal mass range (M_ZAMS < 20 M_⊙) of the progenitor stars, the low-to-normal explosion energy of the SNRs, and the fact that the fraction of SNRs hosting magnetars is consistent with the magnetic OB stars with high fields.