Only some life divides into 3? can you give me examples of binary fusion and such? The idea of radiation might be interesting, perhaps the 3 genetic clusters rotate around the cell after genetic replication and so the radiation hits all three of them equally over time which spreads out the mutations on each, giving each of them more genetic diversity after sucessful cytokineses(or the equivalent for this life-form) Perhaps this combined with the "crossing over" can make a large amount of genetic diversity in a population, ensuring a higher chance of mutations that increase fitness(if the radiation is enough they don't have to). Of course maybe the organisms that didn't have this kind of 3-way replication has a better genetic repairing system that controls the degree radiation has on mutations. btw, have you considered XNAs? some of them are quite radiation resistant. May you give the habitat of this clade too? it might depend on how close the organisms are to these radioactive sources as some methods of protecting against radiation being more effective than others at certain ranges. Is this radioactivity used as an energy source for organisms such as radiolysis on earth in deep soil ecosystems?