I know where you're coming from. I also work from home, and have kids underfoot. Mine are a bit older (10,8,6), but you might be able to adapt this to you:
Create a schedule. Know exactly when you're going to work and when you're going to eat and when you play with the kids and when you do the tidying. I have a weekly schedule that have every 1/2 hour for the week, Sun-Sat, 6am to 11pm. I do it at the beginning of each week - first put in my faith/church obligations, then my family obligations (things that must be at a certain time) and then I plan the hours I can work my business. After that, I will write in the play time and the housework (I even write which housework, like laundry or vacuum). Make sure you include meal time in there too. I heard one woman say that she writes down what they're having for dinner in the 4:00 time slot, so she makes sure to get it ready in time (for her family's 5:15 dinner). Once you've planned your week, stick to it. At first it may feel constraining - you will not see a lot of blank spots and it might feel restrictive. But I found that when I stick to my schedule I feel a lot more free. When I'm working I'm not so worried about what needs to be done in the house or with kids, so I work more efficiently. When I'm playing with kids, I'm not worried about the work I "should" be doing.
Get your family involoved. Put a copy of the weekly plan on the fridge. My kids remind me when I'm supposed to be doing something else, and when it's their time. And your kids can help with the cleaning. The 3 year old can probably pull laundry out of the drier, and help you fold towels and stuff. He can even help unload the dishwasher. The 9 year old can load an unload the dishwasher, help with laundry, vacuum, dust, etc (it may not be as perfect as you doing it, but it's done anyway). They can both put their toys, and even clothes, away as well as help with the baby's things. You'll have to teach them how to do it all, but then it'll take the burden off you.
When the college girl comes to help, close yourself in your office and make sure she understands to not disturb you except in an emergency. You can take advantage of those hours to work very efficiently so you can do 'kid' stuff more when she's not there. (I'm assuming that she comes to help with kids, not as an office assistant). Delegate as much as you can to kids and helpers.
The best thing about working at home is that you're your own boss. But that can also be the worst thing - there isn't the accountability that you would have at a 'regular' job. So become a very strict boss for yourself, and honor your work schedule as much as you would for any other boss. You do have more flexibility with your schedule, and you're available for emergencies, but don't let yourself get distracted over the little things.