Sachs Torpedo 3 gear vs. SRAM T3 vs. SRAM i-motion 3 gear
SRAM acquired Sachs in 1997.

The main visible difference between the (Fichtel &amp;) Sachs Torpedo 3 hub and the SRAM T3 is that the SRAM T3 comes in an aluminium body, whereas the Sachs Torpedo 3 comes in a chrome plated steel body and the brake design has been changed (but is compatible). There apparently was a successor model to the Sachs Torpedo 3 called Sachs Spectro T3 in between the SRAM T3 and the Sachs Torpedo 3. Around 1996 the coaster brake mechanism was changed by Sachs. So the timeline is:

~1977: H3111 (Sachs Torpedo 3) -> ~1996: MH3111 (Sachs Spectro T3) -> ~2000 MH3115 SRAM T3.

For the (Fichtel &amp;) Sachs Torpedo 3 (H3111) speed hub see:
http://www.scheunenfun.de/f+s_h3111.htm (German)

For the Sachs Spectro T3 (MH3111) and the SRAM T3 (MH3115) see:
http://www.scheunenfun.de/sram-t3.htm (German)

So those mentioned above are internally compatible, at least if you're willing to replace some parts.

I am not exactly sure about the i-motion 3, but SRAM states that it is the successor of the T3 hub (see http://www.sheldonbrown.com/harris/pdfs/sram-internal-gears.pdf) it has a different shifting mechanism (a broken cable leaves it in low gear (opposed to high gear on the T3) among and some other differences, as far as I can see (see http://cdn.sram.com/sites/default/files/techdocs/sram-techmanual-my10-gearhubsystems-english-reva.pdf) and the internals are not compatible with the t3.

Some people have had problems (broken axles) with the i-motion 3 hub, whereas the old Torpedo hubs rarely break, which should be the case for the T3 as well, as it is mostly the same design (see http://www.bikeforums.net/archive/index.php/t-394516.html).