Croppy - I know plenty of au-pairs who have sole charge of children, mine included. I can't afford a Nanny, nor nursery nor childminders on a full-time basis for my 3 cherubs. I was an au-pair in the States and I took sole charge of the girls I looked after.
I understand what you mean about 'newborn' but presumably Knakered will not be going back to work immediately after the babe's born. I also appreciate that you wouldn't like the idea of leaving your children in sole charge of an au-pair. It all depends on our individual circumstances, which are as unique as our fingerprints.
Knakered - My suggestion was a 'starting point' you could arrange a childminder for the babe, and then if the other 2 are at school, you'd probably only need one au-pair. You could even explore the possibilities of working part-time/job-sharing etc etc. Think about your priorities what YOU want and try to honour that as best you can.
For me, finances permitting, I definitely would have stayed at home after the 3rd was born - unfortunately we couldn't (still can't) have afforded luxuries such as food, electricity, gas etc if I didn't work. That's why I am praying for a miracle.
Money-Saving tips? All preceded by 'if you don't already': take packed lunches to work, don't go window-shopping, buy EVERYTHING in cash (you may want to spend less - it does feel harder to part with notes than hand over a credit/debit card) and finally (on the spreadsheet side of things) on paper halve your income (pretend as though you had to) and work out what you would have to give up/what changes you would have to make to stay in the black. I tell you after you've done this you'll feel like a millionaire once you realise your true current income.
I wish you all the best with your new arrival.