The third variant is simply wrong.
The first variant is not wrong, but it is not common. Many people will use the present tense after "to ensure" regardless of whether they mean present or future.
I suspect that the reason is that what is normally meant is to ensure that something is true in the present and remains so in the future.
They want to ensure that their children do well in class
They want to ensure that their children do well in class now and will continue to do well in the future.
Careful users of English may use the future tense when what is being discussed has relevance only for the future, e.g.,
One reason that we try so hard to show mutual respect and affection to each other around the children is to ensure that they will have a model for maintaining a strong marriage when they grow up.
But I admit that many native speakers are not careful users of English and that even the careful are not always careful.
The second variant with "do" is so unusual in American English that it is tempting to call it ungrammatical. That temptation should be resisted. There may be circumstances that call for emphasizing what is to ensured with the modal.
We should ensure that every police officer does indeed get training in what are the basic civil rights of even criminals.
To sum up, you will be understood and sound idiomatic with "to ensure that" + subject + verb in present tense." Other forms may be grammatical, but tend to be used in special circumstances and are seldom if ever mandatory. "To ensure that" + subject + verb in simple past, past progressive, or past perfect are not grammatical.