。
The trees are planted three metres apart.
Literal
Tree [topic-は] 3-metre separate-[and-て] has-been-planted.
離す ('to separate, keep apart') is transitive; its te-form 離して functions adverbially — 'separating [them] by 3 metres' = 'three metres apart.' Paired with the passive 植えられている, the overall meaning is 'they have been planted three metres apart.' Both verbs inflect in ~ている: 'has been done' for 植える, functioning as resultative perfect.