Hi sarahmarie,
I am in a very similar position to you, I am 19 weeks pg, and have DDs aged 2 and 4. I am finally over my terrible morning sickness and am feeling physically much better, but over the past few days have had thoughts about the boredom and drudgery of looking after a tiny baby again, especially when I would rather be concentrating on my older 2, as DD1 will be starting school around that time too.
FWIW, I was very lonenly and bored on my first mat leave, but found my 2nd much, much better as I had made some mum friends I could visit/call, plus I found myself so much busier with the 2 under 2s to deal with.
This time I am planning on focussing on DD2, and just making sure I get her out and spend lots of time with her, whilst DC3 will just have to join in. The first few weeks they sleep lots anyway, so you'll be able to enjoy the peace - read, watch tv etc whilst they sleep, enjoy being alone until your older two get back home from school.
I think it's important to plan ahead for this boring time as well - I am going to treat myself to a kindle or an i-pad, some smellies etc, so I can spend the early days of mat leave just relaxing, till baby gets a little bit older and more demanding. I am also finding out about baby classes etc already, so I know my days will include some sort of activity and getting out of the house.
As you have two older ones you'll know what to expect this time, so anything you are able to do now to make that time easier may help you feel less anxious about those first few boring weeks/months?
And failing that we can MN all day long :)