First of all, while I run three guides, I'm certainly NO superwoman! It just works for our family. I think the biggest key is to give yourself A LOT of grace in the beginning and realize that for a few weeks things probably won't run like clockwork. You'll have to establish a new routine for school essentially.
We all stay in the same room until we break or until one or another finishes. Since how to keep an almost-3-year-old occupied isn't in the equation I'll just talk about my older two
I first made a list for each child of everything they accomplished during the day, but just in general terms. Dictation, reading, math, etc. I then though about which items on the list could be done independently or semi-independently. For instance, handwriting is pretty much an independent thing while math is independent only after I teach the concept. I then rewrote the lists into blocked sections for independent, semi-independent, and completely dependent upon me to be teaching. That way I could chunk my time easier. I don't have a set schedule of the order we do our work, but I do have a set block setup so first thing my middle girl is working directly with me, which is most of her stuff because she is in Little Hearts. While I'm working with her my older daughter in Bigger is working on handwriting, Bible memory work, independent reading (We're not using DITHR yet), and math. We're not using Singapore and it is a bit more independent. She also writes in a journal every day (by her request) and I usually have that available if there is time. Once my youngest is finished I work with my older daughter on the history and science stuff. While she is working my other daughter is usually playing, which I think it fine.
I do have a lot of fun learning games we don't pull out very much. I saw a great idea to write them down on slips of paper, put them all in a jar, and pull them out whenever there was free time. It is a great way to use all that stuff that sometimes just sits on our shelf.
I'm happy to answer any other questions you may have

This is my first year running 3 guides so there are certainly days where I'm muddling through! This week I haven't made it to Little Hands yet...hopefully tomorrow!