Memory says it's full but there is 3GB of space not used. Iphone 6 16GB. How to access the

I purchased my Iphone 6 16GB December 2014 and it is great for me but i'm a student and I use it a lot and have a lot of stuff on it. However, I have a big issue with the space and I have to constantly delete apps or other data. I haven't updated to ios 3.2 (it's there not downloaded mainly me not having space) or I don't really icloud properly as in I actually don't use it a lot but it's enabled. But memory on phone says there's but when I count each app there's nearly 3GB of the 11.5 GB that's not counted for?! I firstly don't even need or want 4 gb of Icloud as I backup everything manually. Also, is there a link with me always having say from 50mb - 400mb free whenever I use it as it tends to randomly go slow or a complete freeze or even screen just goes black but it's still on where I have to do a manual reset. So basically, it says I have 11.5 GB of space with 127 MB available but after countring each app data theres nearly 3GB that's not counted for. How can I acess the free memory and be able to use up the 3GB memory of space?