1. My friends are to come back home tonight.
2. My friends are going to come back home tonight.
3. My friends are due to come back home tonight.
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Which one of the two is similar to #1?
Is #1 used commonly?
#1 is very awkward and not commonly used in standard English. The other two are fine.
Out of the three sentences, #2 "My friends are going to come back home tonight" is most similar to #1 "My friends are to come back home tonight." Both sentences express the future action of your friends returning home tonight.
Sentence #1 is a less commonly used construction in modern English. It may sound slightly more formal or old-fashioned. However, it is still grammatically correct and can be used in certain contexts.