I suggest you use InDesign, and forget Illustrator and Photoshop at this point. You're starting with a Jpeg, which is a bitmap image, so whether you change it's size in InDesign or Illustrator, it's quality will go up when scaled smaller, and it's quality will go down when scaled larger. Photoshop has the ability to resample an image when scaling, which means it throws away pixels when scaling smaller, or it adds pixels when scaling larger, but it only estimates what those newly-created pixels should look like, based on the existing pixels in the given area. The results can be better, worse or maybe you won't see a difference at all. You can also scale an image without resampling in Photoshop, which is the same as scaling it in InDesign or Illustrator. Since you're starting with a larger image than you need and scaling it smaller, you won't have to worry about losing quality, and the only downside is that you may have more pixel information than you need. That's only an issue if it leads to excess file size, which translates into longer imaging time or storage space, but that's probably not going to be an issue for you.
I suggest you make an InDesign document at each of your chosen page sizes (A3, A4 & A5), place the Jpeg on the page and scale it if you need. The 2 A5s on the A4 can be set up as A4 in landscape mode (wide, rather than tall), and just place two copies of the Jpeg side-by-side. When you're done, export each as PDF and bring it to the copy centre.
1. I would avoid jpgs as far as possible to avoid loss of quality.
2. Set up a A5 InDesign document, in your case start with page 2, 2 pages
3. Place on each page one flyer
4. Export PDF as spread.
BTW: 45% would be the very wrong scaling factor. Each A-size length is smaller by the factor of the square root of 2 (=1.41421) , it starts with A0 in a size of 1 square meter with a side ratio 1:1.41421 (this ration is for A mainline always the same), each smaller size divides the longer side of the previous A-size by 2 and is rounded to complete mm. Eg. A4=210 x 297mm; A5= 210 x 148mm A6=105 x 148mm; A-Long=210x105mm)