I left DD3 with my Mum when she was about 6 weeks old for the whole day - 8am ish until about 8 pm ish.
It was DD1's 11th birthday, and we promised her we would take her to Luna Park the previous birthday (prior to getting preggas). So we took her and DD2 and a few friends.
I had a little seperation anxiety, but I had every confidence in my mother and knew she would be fine. I couldn't ignore the older girls just because there was a new baby. Poor third child syndrome...it just fits in/tags along lol
I think, do it when you're comfortable with it. If you aren't ready yet, you probably wouldn't enjoy yourself anyway. You could do something special at home with DH until you feel more ready to leave DS for longer periods.
3 girls are enough...the shop is now shut!