You can't really force Java to use up all the memory you give it. It is not the only indicator of performance, and in reality a pretty trivial one if you have 64bit Windows. Java will allocate itself a minimum heap size it is comfortable in if you really did set the maximum heap size to 3MB.
I would look into other aspects of your computer, for example CPU speed, storage devices and most importantly, your graphics card.
You need to install OptiFine. You may think that it slows it down more, but you'd be surprised. I have minecraft at 500 Megabytes and it runs 100FPS. I do have a mac, but I don't think that would be contributing much, as they are not meant to be gaming computers
And what about a chunk disappearing near my castle whenever i get away from it a few steps and coming back when I approach it from a certain direction? that's crazy, isn't it? And I have pasted an old backup of that save and there's no problem with that chunk. So i guess the savegame is corrupted. How could I fix it? I've followed several tutorials and haven't come up with a solution!