Have you tried typing 3/4/14? Or are you saying that as soon as you type 3/4 it converts it to ¾? I know I have some keyboard shortcuts set up on my iPad to automatically convert the most common fractions when I type, such as ½, ¼ and ¾. If I want to type "3/4" instead of ¾, I type "3/ 4", then back up and remove the space.
Glad that helped. The fractions should definitely be in Character Viewer. I often find Character Viewer rather confusing, but in mine (OS X 10.8.5 Mountain Lion), I see the fractions in the "Digits - All" category on the left side of the window. Double-click the desired fraction to insert it at the current text input cursor point.