Since Bethesda has not confirmed a Construction Set for Fallout 3 we can only wait and see whether or not it's coming.
If you're a modder and want to take a at how the Oblivion CS works (as the F3 CS might well be similar), here are some resources:
Official TES Construction Set Wiki.
UESP TES 4 Modding Wiki (more technical).
Official Oblivion Construction Set forum.
If you're a modder and want to take a at how the Oblivion CS works (as the F3 CS might well be similar), here are some resources:
Official TES Construction Set Wiki.
UESP TES 4 Modding Wiki (more technical).
Official Oblivion Construction Set forum.