While the Census Bureau does use more complex methods for generating multi-year weights (see Weighting Methodology from the 2012 3-year Accuracy Statement ), simply dividing by three is the primary adjustment and provides extremely similar values to the Census Bureau calculated weights. You can verify this by trying to match the 2011-2013 PUMS Estimates for User Verification. See this answer for more details on constructing the multi-year files yourself.
The Census Bureau has proposed discontinuing the 3-year ACS files. IPUMS will continue to provide the 3-year files as long as the Census Bureau releases them. Generally IPUMS-USA releases multi-year ACS samples within 2 months of their initial release by the Census Bureau. This year the IPUMS-USA team is working on improving their data creation process. This will ultimately improve the IPUMS-USA project as a whole, but with the unfortunate side-effect of delaying the release of the 2013 multi-year ACS data files. At this time, there is no estimated release date for the multi-year samples.
Hope this helps.