Complex sentence: The sources all agree on that the I-3 is the same as the I-380 (using the old design bureau designation series), but they all disagree on the (old) designations for the I-3P and the I-3U.
Split into simpler sentences: The sources all agree on that the I-3 is the same as the I-380 (using the old design bureau designation series). But sources all disagree on the (old) designations for the I-3P and the I-3U.