Getting coordinates of a device in a 3d space
I actually published a paper on this a couple years back. It is indeed difficult to track objects indoors with an IMU. As mentioned here previously, the only practical way to do it is with updates. I used passive RFID tags to update the position of my very low cost IMU (less than $100 prototype). The IMU works for short periods accurately but will begin to drift, all it needs is to occasionally pass near an RFID tag that has a unique ID and location associated with it. At that point not only do you know your current position, you can perform an affine transform on your previous path to get a more accurate picture of where you were. Your question is a bit vague on your application (I was tracking humans), but perhaps this dead reckoning with RFID fiducial updates would work for you. The paper is called "Indoor localization using pedestrian dead reckoning updated with RFID-based fiducials" if you'd like to look it up. It may give you some inspiration for your own method at least.