Got some fields that are now 6 years soybeans in a row. Don't see any yield drag. No diseases, nothing yield reducing. I think some soils will take continuous beans better than others. My heavy clay seems to do very well. My beans have always been on 30"  ridges. It's the ticket for me. I've had neighbours with white mold and my fields have never seen it, sometimes right beside each other. Their's were drilled. My beans get better fertility.

"A decline in yield in the 3rd year is likely ??" --------- I think that statement is a myth. There are too few people trying it to prove it false. My beans have actually increased in yield every year.