I had the same problem with my 2nd and especially my third pregnancies. My iron was low & by after lunch, I was exhausted! Instead of supplements (I didn't need any more constipation issues....) the first thing that midwives try is to take 1 tbsp. twice a day of liquid chorophyll, it has to be liquid, not capsules. It helps your body "hold onto" the iron you are already taking in. My iron has continued to be low and my youngest just turned one. I still take it. I buy it from the Vitamin Shoppe. I can tell when I have skipped doses. Also, try and eat more protein. That will help your body hold onto the iron too. Your baby needs the protein anyway.
Also, I don't know how old your other two are, but try and nap when they do, or at least get them to rest and you can do the same. You need your sleep while you can get it and you'll be much more ready for your labor if you can be well rested instead of dog tired by the time it finally arrives!
I agree with taking a little walk too. And, are your thyroid levels okay? Sometimes they change during pregnancy and that can throw you off too.
Good luck and enjoy the rest of your pregnancy! If you are still tired and struggling, don't be afraid to ask for help. With the kids, with housework, meals, whatever. And I don't necessarily mean from your husband! I mean from other moms and women that are in your circle.
Blessings~