I would - I take my dd's to WDW by myself fairly often but I know there is a big difference between 2 kids and 3. Considering the ages of your children and that you have been recently I think its very do-able.
I would imagine that you do many things with all 3 of your kids as the solo adult so think of this as just another trip to the zoo, mall, supermarket etc. = just a little longer
The nice thing is that you don't have to cook or clean and you can really spend time just having time with your kids.
A few things to think about; one is the quick serve meals and buffets - can one of your older kids sit with the 4yo while you and another of the older kids get qs meals? I would consider getting a grocery delivery and a cooler so you can have some favorite foods and snacks in the room and save having to run to the food court as often. I use wegoshop.com and have had excellent service.
Parades/fireworks - be prepared that no matter how much you plan and grab a spot in advance, someone will probably have to go and you may loose your spot. I brought along a beach towel to help save spots for parades and fantasmic. If I did have to run on a bathroom trip with one of my dd's the beach towel helped the other child hold a spot. Other times we just all left and found a new spot after.
I would make sure that you have a pretty good but simple plan for each park outing. Its going to be hot in August so maybe have everyone pick 1 must do and after that everything else is a bonus. We are going the 20th of August and plan to hit the parks at opening, swim in the afternoon and return out for evenings at the parks. We also have a waterpark day planned.
Maybe all of us solo "parent's" who are going in August should meet up - perhaps we could have our own grand gathering - JK
TJ