This is something I've done a lot of research on and getting timelines ironed out. Sony did sell the 3.5in Micro Floppy Disk drive before Matsushita and Hitachi were able to get the 3in Compact Floppy Disk on the market, but only by months, not years. The Amdek Amdisk III was being shown in 1982 [1] using 3in CFD drives. It was being reviewed by publications in 1983 [2]. The drives were on the market before Amstrad used them and were very seriously contending with Sony [3].

The Sony MFD had more industry support than the CFD, but it didn't hit critical mass until the Macintosh released in 1984. Every single machine before that using Sony MFDs failed to widely compete because the drives and disks were still too uncommon, aside from HP who had a closed ecosystem. Some (Jonos Escort, CDI DOT) even attempted to re-released with 5.25in drives to save the base computer design. In 1984 there were still new [4] small floppy formats being introduced, it absolutely made sense to still try to use CFD. I would say especially for a lower end market aimed at children CFDs would be better as they are more robust.

Why Amstrad chose the 3" drives