I've tried to find out more about the Soviet use of what seems to be the Fairbanks-Morse engine from my university library. Not much help, and some things just get more mysterious!
One thing I found was a 1957 pamphlet (64pp) on "Transport Developement and Locomotive Technology in the Soviet Union," by James H. Blackman, published by the University of South Carolina (Bureau of Business and Economic Research, School of Business Administration). No technical details on the locomotives, but an interesting sidelight on the economics of dieselization: it was estimated that either dieselization or electrification could about DOUBLE the capacity of an existing Soviet rail line. I suppose the idea is that signalling and siding spacing, etc, limited the NUMBER of trains that could be put through in a day, and that multi-unit diesels could double the SIZE of an individual freight train over what the steam locomotives then in use could handle.
More to the point, J.N. Westwood has written two books on Russian railroads (as well as other books on other Russian topics). One is a general "History of Russian Railways" (Allen & Unwin, 1962), which doesn't help on our question. The other is "Soviet Locomotive Technology During Industrialization, 1928-1952." This has lots in it, much of it sad: much of the design effort in the steam era went into locomotives that were theoretically very efficient but too complicated to be maintainable in practice. ... Markovich, one of the designers of the legendary AA20 (the 4-14-4 steam locomotive) was accused of Trotskyism and sent to work in a locomotive depot in the Urals after it failed. ... Soviet railroads were world-pioneers in dieselization... but the pre-WW II designs were clunky and seem to have had no influence on postwar designs, which were inspired by American practice (and in particular by the Alco RSD-1)! There was pre-war technology transfer: Russia acquired a few electric locomotives from GE (which apparently formed a basis from which significant later Russian electric locomotive designs evolved), and also a handful 2-10-4 steam locomotives from Alco and 2-10-2 from Baldwin, whose detail design had an influence on the main Soviet late heavy freight steam design, the FD type 2-10-2, which in turn influenced the Chinese....
The TE-3 locomotive we are interested in was introduced at the very end of the period covered by the book. (There is a 1956 photo of an erecting hall at the Voroshilovgrad works with TE-3 under construction at one end and LV steamers (a lighter 2-10-2 design than the FD) being finished at the other!)
On the internal anatomy of the TE-3, Westwood says:
"It employed opposed-piston diesels (said to be derived from American Fairbanks-Morse engines used on small craft received by the Red Navy during the war)."
Which, I am afraid, is the nearest thing to "documentation" I've found on how the F-M technology got transferred to the USSR. Note that Westwood is not a naval historian, so his use of the term "small craft" is perhaps loose. The two best-known United States Navy applications of F-M engines during WW II were on submarines and destroyer escorts, and no vessels of these types were supplied to the Red Navy under Lend Lease. On the other hand, three American-built icebreakers (not exactly "small craft" at 289 feet long!) were lent to the Russians in the early 1940s (returned to the U.S. about 1950). These were of what seems to be called the "Burton Island" class (why I don't know: Burton Island herself was not the first of the class), and each was powered by six ten-cylinder F-M engines. Perhaps one of these spent time in a Russian shipyard under "repairs" with Soviet engineers carefully reverse-engineering blueprints from its engines! But I can't say for certain.