Sam, thanks for the info. I think the 3 holes are not to provide lubrication to the chain, but to vent the crankcase pressure into the primary case. I always thought the level of the oil in the primary was what mainly lubricated the primary chain. Also, (see new photo) there is an oil line that is directed at the chain, located near where the stator wires exit the case.
Re the highlighted text above - No. The 3 holes are oil drain holes and they are too low.
Your engine case have been modified from the earlier, timed breather system to the later open breather system.
Originally your engine would have had an oil seal adjacent to the drive side main bearing to prevent oil from the crankcase migrating into the primary chaincase which would have had it's own oil supply entirely separate from the engine oil system.
From 1970 onwards the timed breather was discontinued. The oil seal adjacent to the drive side main bearing was deleted and the engine breathed through the main bearing, into the primary chaincase and then out through a pipe which runs to the rear mudguard. This means that the crankcase and primary case are connected and so any oil blowing through the main bearing would eventually overfill the primary case. To compensate for this the 3 small holes were drilled to let any excess oil in the primary case drain back into the crankcase. The oil line which is directed at the chain merely collects oil that is flung off the clutch assembly and drips it onto the chain.
Because your 3 holes are too low, the chain will run dry as the oil level will below the lower chain run.
As Stuart has mentioned, your cases have been modified to run a dry, belt primary drive. DO NOT use a chain unless you plug those 3 holes and drill them higher up as per the picture I posted above.