Yes, Redmi 6A does have 3 slots - 2 for sim cards and 1 for the MicroSD card.
No. Redmi 6A has 2 dedicated sim card slots and a dedicated MicroSD card slot. So, you can use 2 sim cards and a memory card at the same time.
Yes, Redmi 6A does have the GPS support.
Yes, Redmi 6A does have the night mode. It also has eye protection mode.
The Redmi 6A is a fairly good update to the Redmi 5A, which itself was barely any different compared to the Redmi 4A. You now get a tall screen, better processor, and improved polish overall.