Well, I was surprised to read about you:
My A3 2017 SM-A320FL (Exynos 7870 arm64) definitely has an A-Only system partition and was flashed with a corresponding image. Please check it again with Treble Compatibility Checking App ‘Treble Check’.
For my part, it is planned to share my experiences with Samsung Galaxy A3 2017 SM-A320FL and @harvey186’s [GSI] Q-GSI build 10/11/12/ 2020 here in this /e/forum.