If you really want to make an image fully fit a Pp thumbnail you have to make it square.
Most images start life as rectangles, either in portrait or landscape orientation. To make them square you have to change the shape. Which will either involve cropping top/bottom or sides (depending whether it's portrait or landscape originally), from a rectangular image, or adding blank areas to the top /bottom or sides. It's not difficult to do in something like Photoshop.
But I wonder if it's what you mean?
When I post something once I've uploaded the (rectangular) image I slide the little 'walking ants' square frame within the rectangular boundaries of the original image. Which gives me the best (well best IMO) square window within my original image shape.
I think of resizing as something completely different to changing the shape. In terms of your 3Mb target, I suspect that you'd be best off going for 1080 wide instead of tweaking the dimensions so the upload is ♥Mb. I think at 1080 width, the site says it doesn't change the image quality on display, whereas any other width will be modified by the output process (someone might want to comment as to whether what I'm saying here is accurate)