Thank you guys so very much. With the assistance of your information I think I've finally pegged down my ancestors military service records. I knew absolutely nothing about this man just a couple of weeks ago. Based on your guy's clues and M E Wolf's military document he posted, I was able to peg down that the 2nd photo was taken after 1898, based on his buttons and his Corps Badge.
A special thanks to Johan Steele for pointing out that the third photograph was a Union Telegraph Company uniform and was civilian in nature.
Since that photo has Chicago, Illinois as the location for the photograph I started to speculate that he probably settled down and lived his life in Chicago after his military career. I started searching online for military pensions that were filed in Illinois using a name that was written in pencil on the back of the 2nd photograph. About 20 records came up for that time period. Once I scrutinized them further, only one record really fit my criteria of the age I was approximating for him and the approximate dates of his service time.
After following this lead further I found some more of his military records online. I was able to find a record for a leave of absence the man took. The city he was visiting is a very small city in which I have family ties to. Once I saw this, it basically confirmed everything for me and proved to me that I was following the correct trail.
So far I've learned that he served in the 19th Infantry Army E company and he enlisted in 1895 and that he served in the 7th Infantry G company later on. I'm also starting to think it's the same man in all three photographs I posted, because in the first photograph you can barely make out that the hat insignia is 19 E.
Online, I found a record that shows that while he was in the 7th Infantry G Company, around June 1899 he was garrisoned to the Columbus Barracks, in Columbus Ohio. The record also says he was a corporal at this time. This is likely when the 2nd photo was taken because it was photographed in Columbus and on the back of that photo in pencil it states he's a Corporal.
Anyways, sorry for the long winded response, but I wanted to express my gratitude towards everyone that posted.