Both times I've kept the older girl/girls in daycare a few days/week when the newest one came home. This kept them in a routine, and since they love daycare, it was an easy decision. It also allowed me one on one time with the newborn, and to get rest/things done around the house so that when the older ones were home I could be more 'present' with them.
If you can afford it, I highly recommend it (I kep DD1 in daycare 3 days/week until DD2 was about 4 months, and with DD3, DD1&2 are still in daycare 2 days/week until the end of this month - I am going to miss the time I get with them there, but financially it's a stretch to keep them there). I'm in Canada, so 1 year mat leave each time.