While I'm unfamiliar with the Fallout 3 armor, I can tell you how it's generally done for a Mjolnir Mark VI suit, and how I did it for mine.
First, you make sure the legs are padded so that they have a slightly snug fit. This also keeps them centered around your leg. For the Spartan armor, the codpiece is attached to a utility belt, which is then connected to some type of suspenders so that the weight is being held up by your shoulders, not the waist. Then it's simply a matter of attaching nylon straps and plastic side-release buckles to connect the leg pieces to the belt piece. Place them wherever you feel is the best location.
Feel free to check out my Mark VI build in my sig. Towards the end of the thread, I show in detail how I accomplished this. Hopefully it will help you successfully attach your armor. Good luck!