You can absolutely make 1 decoction with all 3 herbs. I’d recommend doing equal parts of all 3 herbs. So, you could do 2 tbsp of each herb decocting in 5 cups of water. You will have leftovers, but that’s okay! Then use 500mL of that for your herbal iron syrup.
You can absolutely reduce the molasses quantity. I’d recommend adding some brandy, or even a simple herbal tincture you have on hand at home, to increase the shelf life. I’m assuming you’re omitting the honey entirely since you’re including the berries in your decoction? Most syrups are equal parts liquid and sweetener, so yours will be a bit different in consistency, taste, and shelf life, which is why adding some alcohol or tincture will help.
For dosing, I would stick with 2-3 tbsp a day away from other foods/vitamins. You could continue this for 1 month, but I would honestly recommend 2-3 months if you’re very low iron and you’re in postpartum. An additional iron support to the syrup would be making long infusions of nettles, oats, and raspberry leaf to drink everyday. 🙂
For the shelf life, if you don’t add any alcohol in, and it’s low sugar, I would use it within 2 months. If you do add alcohol, or it’s a high sugar syrup, it can last up to a year. Make sure to date it when it’s ready!
I hope this helps. Let me know if you have any other questions.