“So, is there a plan?” wonders a skeptical teen in the backseat of a cramped car on the school run. With these adults, it’s hard to know. Questions of intentions, schemes and the invisible hand of fate animate Rebecca Miller’s witty new comedy Maggie’s Plan, a charming three-hander with wonderful turns from Greta Gerwig, Ethan Hawke and Julianne Moore as inter-linked lovers whose hearts can’t seem to sit still. Gerwig dials down the quirk as the titular Maggie, a single New Yorker eager to take control of her life. She still delivers lines with that air of playful, slightly affected,…