1. He is good at imitating other people's voices.

2. Doing voices is his specialty.

3. She is good at doing imitations.
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Are they all right expressions? Which expression is commonly used?

All are correct, yes. And each would be used about as often as the others.

=)

Yes, all three expressions are correct and convey the same meaning. However, the most commonly used expression among the three is "He is good at imitating other people's voices." This expression is concise, clear, and commonly used to indicate that someone has the ability to mimic or reproduce the voices of other individuals.