If I understand correctly you are fitting a Discovery 300TDi rear axle with the three bolt flange to a Defender 300TDi that uses a 4 bolt flange. In which case you won't need a new propshaft, just change the flange as described above.
However if the axle is staying on the Discovery, when you change to the 4 bolt flange, you would need to replace the propshaft as well so that it has the correct 4 bolt flange from a 200TDi Discovery.
I hope this helps
Pete.