Stacy Keach will return to the role of Richard Nixon for three perfs in the national tour of “Frost/Nixon,” currently onstage at the Ahmanson Theater.After Keach suffered a mild stroke last week, his understudy, Bob Ari, took over the role of the former president in the Peter Morgan-penned play, helmed by Michael Grandage. Keach will return for the Friday, Saturday and Sunday evening performances. Ari will perform Tuesday night and during the weekend matinees. The Ahmanson leg of the tour concludes with the Sunday evening show. According to a Center Theater Group representative, Keach will finish the rest of the tour, which travels to cities including Tempe, Ariz.; San Antonio; and Sacramento, but his performance schedule is undetermined.