Hi Aev.
"If I select "designed at" options as 3x, then 3x image size has to be 1125x2436 ( 375x3, 812x3) right?"
- No. Your "designed at" should be set to 1x. The software will take that variable and export the other two (2x and 3x).
If you set it to 3x, it will take your design and export the 3x from it 1:1. And the 1x and 2x are smaller, so it will divide them from your 3x "designed at" resolution.
When you set it to 1x, then Xd exports the 1x from it 1:1, and then multiplies it for the bigger 2x and 3x resolutions, respectively, which is what you want.
I know it's a bit confusing, hopefully that clears it out 🙂