Summary

Designed to fit one Seagate Backup+ Hub and two Seagate Expansion Portable drives. Can probably fit a wider range of two 2.5" and one 3.5" external drives. The Backup+ Hub is nice to have since it has a built in USB hub for the two smaller drives. There's margins for the 3.5" socket (see image) to make room for slightly varying casings of different models. Please let me know if it works if you try with other 3.5" drives!

Backup+ Hub: (depricated)
Expansion Portable: https://www.seagate.com/gb/en/products/external-hard-drives/expansion-external-drives/