Hi @PS.2 ,
either SDMMC1 or SDMMC2 with any of their available pins is possible for eMMC boot.
SDMMC3 is not available as boot source.
Default signals are SDMMC2_CLK (PE3), CMD (PG6) and D0 (PB14) for eMMC boot.
note that as only one data bit needed during BootROM boot, D1 to D7 are freely defined in device tree for TF-A/uBoot and Linux usage.
please have a look to :