Interesting question. Here's a thought:
Perhaps very early in this world's evolution, the first RNA could have combined to begin the process, then very early on a third contributor developed. This became the successful combination on this world, and from there all life developed.
As we recognize two reproductive sexes, male and female, your world would have three recognizable reproductive sexes; eg. male, female, and perhaps "somale". Where we each contribute half of our chromosomes to produce our progeny, they could contribute one third, or perhaps one contributes half and the other two contribute one quarter each. Or, perhaps two contribute half each, with one acting as a "catalyst" of some kind.
The reproductive cycle would be interesting! Would the nuclear family have three parents, be polyandrous, or be very similar to our own but with the "somale" members of society drifting in and out of relationships much more casually? If the latter, would the "somale" gender be less likely to occur? What societal status would the "somale" enjoy if that were the case? Would coitus between the three genders need to be simultaneous or just close in time? If simultaneous, I think the "inseminee" would probably have evolved a second sexual opening very early along the timeline, assuming that semen is even involved.
Or, perhaps, they're "egg-layers", in which case the physiology could be different altogether.
On Earth, plants which reproduce sexually sometimes require two separate genders of plant, but sometimes the plant can reproduce with itself, and sometimes the plants are all unisexual but cannot reproduce with themselves.