A promising modality of radiation therapy is FLASH: a technique in which the full radiation dose is delivered in just 1/10th of a second. This modality has been proven successful in destroying cancerous cells while sparing healthy tissues. The aim is to develop an accelerator to treat large-volume and deep-seated tumors using high-energy electron beams in the FLASH modality. Specifically, we are designing a steady-state magnet that guides three distinct energy beams into three separate beamlines and ensures dose conformality within FLASH timescales. This paper presents a design method that incorporates beam optics and magnetic parameters into a numerical optimization process. The method is applied to the design of a magnetic spectrometer with a varying pole profile. The magnet performance is compared to pure dipole and combined dipole-quadrupole designs.