K.,
I admire you for opening up and asking others for advice. You obviously love your children and are concerned and want to make a change. It is NOT to late to change your discipline ways or for your children to make changes, too!
Since you do have older children (10,8) it might be easier to have a conversation with them about changes you want to make in your family. One of the first things I would do is admit that the way you have talked to them isn't what is best and that you want to be a better Mommy to them. I would develop a list of rules you would like to have in the house (ie: no yelling or screaming at each other, follow directions, simple chores each one must do, etc). Think about what you want your house to be like and make the rules accordingly (even bedtimes for example). Even your 4 year old will be able to adapt.
Then after you post a family rule chart, you also have to have consequences to follow if the children "break a rule." This may include taking away favorite toys, losing privleges, etc. The four year old might be a good candidate for "time out." If the kids know what you want the house to be like and know consequences it gives the a sense of security. Now they might not like it at first, but if you stick to your guns and you model the good behavior (like talking in a firm, but respectful voice) then the children have a better chance of following your lead.
If you can, have the children involved in making the house rules so they feel a sense of ownership in it. You will also have to have a LOT of praise for when they answer you respectfully, do what is expected, etc. They need to hear what they did specifically that was good. Ex: Sarah, I LOVE how you set the table without arguing with me. Wow, I'm really proud of your good choices! Thank you!" (Big hug could follow).
It is very hard not to lose your patience with three children, but the fact that you want to make a change, is HUGE! Best of luck!