If you only intend to use nodes you can also use node references: https://drupal.org/project/references
Which has been around a lot longer and maybe easier to set up. I would create a Showtime node type that has 2 x node reference fields. One to reference movies and the other to reference theaters.
Once you have created your node types you should migrate movies and theaters first and then you can reference them when migrating showtimes. You can do this when mapping the node reference fields. Migrate will map the old item id to the newly created node id. Have a look at this documentation (https://drupal.org/node/1133448) under the subheading sourceMigration.
Personally I wouldn't force the nid's. There is no need Migrate will create mapping tables so you can find out which item maps to which node anyway.