Hi there.
I've only had one baby...but I have several friends with multiple babies. Only one or two of these women look great... like they never had kids. But most of them have very different bodies then they did pre-multiple baby.
I know after just one my body is MUCH different. I would say that yes, a third baby could really affect your muscles that much. I mean... you grew THREE people in there! Not all at once...but over time!
My first question would be do you still have extra weight. If so, I'm guessing a lot of it is concentrated in your middle region. If not, then a couple of things could be going on. 1) stretched muscles and 2) stretched skin.
The muscles you can tone. Core exercises. Pilates, crunches (If you have an exercise ball, do these laying on the ball. The further back you 'roll' on the ball and have to come up, the more you will feel it.) In addition, making good food choices. Lots of vegetables, fruit, a good amount of protein and healthy fats.
Getting out and moving. Walking is great. You can do with your kids and the weather is nice these days for a good brisk walk. But, any extra movement will help.
If there is just stretched skin ... some of it will tighten over time.... but I hate to tell you not all of it will. That would have to be surgically repaired if you so chose.
Just remember, we do have the responsibility to make healthy choices (our bodies are not our own but The Lords). As a Christian mother, you know this already. We also need to set an example for our kids. BUT we also have to remember that our bodies will change over time and regardless we are beautiful.
I hope this helps.