"Mr"

I don't think it has a plural form. But using "gentlemen" in place of that may help.

Thank you for your answer. I think the plural form of "Mr" is Messrs.

"Messrs" is the plural form of "Mr." However, it is quite formal and used in limited contexts nowadays.

Have a look here:
http://www.oxforddictionaries.com/definition/english/Messrs

Which one is correct?

In general I thank all engineers and staff beginning with Mr....., Mr. .....and Mr....... or
In general I thank all engineers and staff beginning with Messre, ........,.........,......and ........

Thank you teechr.

I wouldn't use "in general" in that sentence. In fact, the usual thing is to use "in particular" when thanking people.

In particular, I (wish to) thank all our/the engineers and staff, beginning with Mr ...., Mr .....and Mr ......

Thank you teechr.

You're welcome.