1. She went to the theatre at 6 p.m.
(At 6 p.m. she went to the theatre.)
2. I saw Sam and Sarah at school today.
(Today...)
Предложение "Sam and Sarah saw I at school today" грамматически некорректное.
3. Kim and I are playing tennis at the sports centre tomorrow.
(Tomorrow...)
Время может быть указано в начале предложения, но в конце лучше.