Sienna Miller has opened up about the realities of raising a teenager, toddler and newborn all at the same time . During Thursday’s episode of The Tonight Show with Jimmy Fallon , the 44-year-old actor, who welcomed her third baby last month , was asked about the difficulties of parenting. “I would have said the teenage girl would be harder to juggle until the transatlantic flight I took yesterday with the toddler and the newborn,” she replied . “The toddler now wins hands down. It was an absolute disaster. There’s no negotiating. “And then the looks people give you on the flight. I was like, ‘Oh my god.’ The baby’s screaming, [the toddler’s] screaming. I got to the line, the immigration line, and just broke down.” In addition to her newborn — whose name and sex have not been publicly revealed — Miller and her 29-year-old boyfriend, Oli Green, have a two-year-old daughter whose name has also been kept private. The American Sniper star also shares her 13-year-old daughter, Marlowe, with her ex Tom Sturridge.…