Sienna Miller will replace Lindsay Lohan in a steamy film threesome with Keira Knightley.The British actress will take Lindsay's role as Caitlin - the wife of Welshpoet Dylan Thomas - in new movie 'The Best Time Of Our Lives', written by Keira's mother Sharman Macdonald. Lindsay reportedly pulled out of the project when she was unable to reach an agreement with the film's financiers. Keira is set to play Vera - a childhood friend of Dylan's who is said to have had a sexual relationship with him and Caitlin. Matthew Rhys, who will play the hard-drinking poet, said: "There will be a bit of a ménage a trois scene going on. I hope I'm up for it!"Speculation that Lindsay had dropped out of the movie surfaced last week,when the actress failed to appear on set to start shooting. Filming is set to resume with Sienna in Wales early next month.