I'd suggest skinless bone-in chicken thighs, as they have plenty of fat and collagen to keep them moist and tasty. I've cooked them in French-style wine-based stews, not to mention cacciatorre, for 2-3 hours before now and they just fall off the bone.
It is virtually impossible to overcook them, unless you boil them mercilessly for hours. Just get a nice gentle simmer going - not only will this make the meat tender, but it will improve the flavour of the tomato sauce as well.
Do not use chicken breast - it is far too lean.
It is fairly atypical to cook large, bone-on pieces of chicken—including breasts—directly in soup. After all, who wants to find bones in their soup? (Making stock is another story....)
Bone on chicken breasts difficult to cook evenly, due to their shape. Some parts are thin, and will cook through rapidly, and other parts are thick and require a much longer time to cook. This makes it easy to have overcooked and undercooked areas. They are also a little unforgiving in soup if overcooked, as they tend to become tough or stringy in texture.
Normally, chicken would be pre-cooked via another method, and then cut up or shredded, and placed in the soup in the last few minutes in order to heat through for service. My favorite method of preparing chicken to use in soup or other recipes is to roast it, still skin on for flavor, but any method will do. The skin can be removed prior to adding the chicken to the soup, if you desire.
Note that this method will not add the flavor and gelatin (for that rich texture) from the bones to your soup. Instead, as cook chicken parts for other dishes, save the bones and other scraps in the freezer. When you have several pounds (a couple kilograms), you can use them to make chicken stock. Homemade chicken broth is a fantastic base for soup, and will help you achieve an excellent flavor.
If you absolutely want to cook chicken breast directly in your soup, I recommend that you de-bone it. Cut it into bite sized pieces. As one of the last finishing steps in preparing the soup, reduce the pot to a simmer rather than a full boil, and drop in the chicken pieces. This will permit the chicken pieces to poach gently in the broth. Your soup is ready for service. once they are cooked through—my guess is about 15 minutes, depending on the size of the pieces, but check them occasionally.
Best Answer
Operating an oven when no one is home is not a good idea, although some people would disagree with me.
If you can find an appropriate slow cooker recipe that you like, that would be your best best bet. Dark meat chicken is well suitable to braising, which is what slow cookers are best at.
I cannot give you an exact temperature, as most slow cookers or crock pots have only "high" and "low" as settings... the real issue is leaving the chicken in long enough, but not so long it turns to mush and gets that overcooked taste. Your three hour interval is certainly long enough to be safe.
The hardest part is finding a recipe suitable for your three hour time interval. Most slow cooker recipes are designed for around 8 hours, so you can leave them while at work, although I suspect they would actually work very well in 3 hours--you could certainly try them and googling them is easy. I actually suspect most 8 hour slow cooker recipes are badly overcooked, but haven't done experiments to find out.
Many more traditional chicken braising recipes are much shorter; again, they can probably be adapted to your slow cooker at three hours as dark meat holds up well to longer cooking.
By googling three hours chicken legs, I was able to find some promising recipes, including this one for very basic "barbequed" chicken legs. That could be adapted easily for almost any sauce you like.