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I am calling to regard one of my client's application.

I am calling in regard one of my client's application.

I am calling for regarding one of my client's application.

Can someone help me with this grammar? Which one is correct?

Many thanks for help.
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None of them are right. The easiest thing is to say:

"I am calling about one of my client's applications."

You can also say:

"I am calling regarding one of my client's applications."

"I am calling with regard to one of my client's applications."

The versions with "regard(ing)" sound more business-jargony.

Note "applications".
Comments  
Also, this from Random House Dictionary:

—Usage note

Although sometimes considered poor substitutes for 'about' or 'concerning', the phrases 'as regards', 'in regard to', and 'with regard to' are standard and occur in all varieties of spoken and written English, especially in business writing: 'As regards your letter of January 19'. … 'In regards to' and 'with regards to' are widely rejected as errors.