It is being reported that some large arable farms are considering abandoning the basic payment (the successor to the single farm payment) because of the 'three crop' rule: Three crops
I do think that this rule is a typical example in the CAP of a possibly laudable objective leading to a policy instrument that is deficient. It arose out of a desire to curb the landscape and biodiversity effects of monoculture. However, at one time there was an implicit view in the EU that some parts of member states would be farmed in a way that maximized productivity. Requiring farmers to grow three different crops undermines this and, in my view, is an unwarrantable intereference in their freedom to make their own commercial decisions. I would also question whether it really achieves that much in the way of 'greening'.
Whether farmers would give up the basic payment is an interesting quetion. It can be a very substantial amount for some large-scale arable farmers, but others receive relatively small sums. However, in many cases it is the difference between making a profit and making a loss. The real hope must be that some progress will be made in reducing the impact of this policy instrument.