Land area in a survey number is less than what it should have- taking advantage of this legally

We have some dispute regarding the land in a particular survey number. Total land in the survey number should be 14 acres, but when the land was divided among 3 brothers, only 10 acres was available(probably it was not measured properly during land survey conducted by Govt decades back). After sharing the available land equally(3.3 acres), the additional 4 acres of unavailable land was still registered just to adjust all the land in that survey number to 2 brothers(brother 2 and 3). Now brother 1 has 3.3 acres, brother 2 and 3 have 5.3 and 5.3 acres of registered land though the actual possession of land is just 3.3 acres each. This status continued for 30 plus years. Now brother 1 sold his land(3.3 acres) to brother 3, now brother 3 has 8.6 registered land(6.6 actual possession). Then brother 2 sold 3.5 acres land to some X(though he has only 3.3 acres of actual land possession). Again brother 2 has sold 1 acre to some Y (actual land is not there at all). Now brother 2 still has 1 acre of land still on his name legally. Now Brother 2 (claiming his 1 acre land is completely occupied), x(claiming that land is 0.2 acres less than it should be) and Y(claiming his 1 acre land is completely occupied) together sent legal notice to brother 3 saying that brother 3 illegally occupied their lands and went to court. What should be the legal approach of brother 3? What happens usually in these cases ?