This question is restricted to post-WWII submarines designed to have a greater speed while submerged and to cruise underwater (unlike WWII U-boats).
As highlighted in this question, there is no need to have 2 rudders (one on the upper side, the other on the lower one) and 2 fins for pitch control (stern plane?) (one on the left and one on the right side). As I don't know the exact vocabulary, for the rest of the question I'll name all those fins "rudder", whatever axis they control.
As highlighted by the Zeppelin NT, 3 independent rudders offset at an angle of 120 degrees are enough to control rotation along all axis on a vehicle based on equality between buoyancy and weight. (Note that with 2 independent fins, you can have a 2-axis control (e.g. roll and pitch)*)
I see several possible advantages of using fewer rudders:
- fewer links between rudders and the hull, easing stealth (less hydrodynamic interaction, smoother shape)
- shallower draft (even shallower than X-shaped discussed in that question) **
- maintenance costs (fewer moving parts)
Of course, rudders' dimensions should be adapted to provide enough authority.
As I failed to find submarines with fewer than 4 rudders, I would like to know if it has already existed.