I tried to translate what I could. Unfortunately my German (and English as well) is still not as good as I would wish to - but I guess at least something might be a good starting point for you. Hopefully, someone else might be able to help you with the last letter which is written in Kurrent.
The General Militär-Kasse [military cash office] was today authorized to pay out 6 Marks belonging to your deceased father, Lieutenant in reserve Krause, as an honorary bonus for July and August 1917 (2 x 3M). The payment will be carried out after presenting officially authorized receipt.
Letter 1 (above the photo)
please forgive me that I haven't responded to your postcard yet. I have a lot to do at the moment – quite by myself without my friends. “Lehrl” and Baumgartner are still in Vienna. [I am unable to read and translate the following sentence completely. It mentions that “Lehrl” had some business dealings there.]. Mayer Franz is now in Vienna. I will let you know the address later. What is with your holiday? Let me know, when you will be on the way, and we will meet in “Ebenthal”. Now it is about time for me [?] the quickest [?] in [to?] civilian life.
With best regards, your friend Johann
Note: The last sentence seems to me a bit unclear. Perhaps it is fault of my clumsy German; perhaps is there a missing word or two. As far as I dare to guess, it looks like that the author could be very close to leaving the military service.
No problem, I am always glad to help. :-)
I gave the last letter another (not much successful) try, but at least a small piece of information came out.
It is addressed to Miss Klärchen* Bauer living in Berlin Schöneberg and it begin "My good darling". Besides there are two places in the text which seem a little familiar: "is your brother" and then (in the very end) "have now no time".
* Klärchen is a very friendly and nice form of the name Clare; such forms are usually used between people with a close relationship (siblings, lovers, etc.).
I hope it might help a little.