I don't see any need to turn engine No 3 round. Engine No 2 is receiveing torque through the nose of the crank already, and I can't think of any lack of symmetry in the crank design that would make it good for one direction flow of torque only. The crank nose is no different to the rear stub flange onto which the flywheel or torque convertor is bolted - you just need a different adaptor flange to couple the outgoing prop onto. So you simply finish up with a coupling shaft between the rear of No 2 and the front of No 3 that carries a set of chain wheel sprockets to drop the drive down to the new transmission axis below the chassis.
To be fair, this does mean that you have to replace the extension shaft / front pulley with something different to couple the prop up to, because the existing set up is driven soley by the keyway, which is probably not up to the full torque of a V8! A good excuse to lose the water pump and substitute with an electric one while you're at it!
If a satisfactory means of transmitting torque through the nose of the crank proved impractical, then there's no reason why you shouldn't simply extend the principle and have a similar coupling shaft and sprocket between engines 1 and 2 and have a third sprocket in place of the flywheel on no 3. Then all that the nose of the crank is doing is to maintain the three engines in syncronicity rather than carrying serious torque. Downside is that you have a long shaft running underneath all three engines to carry the three sets of sprockets and transfer the power into the gearbox. Up side is that you can use the chain drives to drop the rpm - say 6,000 engine rpm equates to 1,500 shaft rpm, which would allow you to use a bus transmission without having to worry about the input speed.
Chris