OK, A3 size has an aspect ratio of 1.4142. Your first image has an aspect ratio of 5747/3472 = 1.6552, so it's a bit longer and narrower than A3; your second one is 4789/3465 = 1.3821 so it's a bit squarer than A3.
Do you want to crop the images to exactly A3 shape? Or do you want to maintain their current shapes and print as large as possible on a sheet of A3? What you wrote wasn't exactly clear to me.
If you're happy to crop them both to A3 shape:
* The first one is too long so you need to crop the long side to 3472*1.4142 = 4910 pixels. Then it will be 4910*3472 and it will print to A3 size at 297 pixels per inch (ppi) so you'll get great results.
* The second one is too wide so you'll need to crop the short side to 4789/1.4142 = 3386 pixels. Then it will be 4789*3386 and it will print to A3 size at 290 ppi - again, it should look great.
If you want to keep them at their current shapes: remember A3 is 420mm by 297mm.
* The first one will fit the full 420mm length of A3 but its width will be only 420/1.6552 = 254mm. So you'll have to trim the width by 43mm.
* The second one will fit th full 297mm width of A3 but its length will be only 297*1.3821 = 410mm. So you'll have to trim the length by 10mm.
