I have twins and two infant carriers ARE neccessary... Graco has a great double stroller; I recommend the one that is front and back rather than side by side b/c the side by sides do not allow the seats to snap in. I know they make a stroller similar for triplets; I would recomment that so she can put the 18 mo old in there too!
If you have any other questions, please let me know... Twins are amazing and a lot easier than people lead you to believe! Mine are 16 weeks and sleeping 12 hours at night already... BIGGEST KEY... FEED AT THE SAME TIME - SAME SCHEDULE IS HIGHLY IMPORTANT...