It could be an ancient sage. In Naruto lore, there are sages who have had a great influence on the world. An ancient sage could be the one who created or fathered the bijuu. Maybe he had some great power and knowledge that allowed him to bring the bijuu into existence.
In many Naruto fanfictions, if Naruto has three bijuu, they could be the Nine - Tails (Kurama) which he already has in the original series, perhaps the Eight - Tails (Gyuki) as it has a connection with Naruto's world and Akatsuki's hunt for the bijuu. And another could be the Three - Tails (Isobu). However, it really depends on the specific plot and creative direction of the fanfiction.
In fanfiction, if Naruto is half - bijuu, it could mean he has enhanced physical abilities. For example, he might have greater strength and speed like a full - bijuu. His chakra reserves could also be much larger, allowing him to perform more powerful jutsu for longer periods.
One possible plotline could be Naruto struggling to come to terms with his half - bijuu nature while also dealing with his relationship with the fem bijuu. Maybe there are other characters who are either afraid or in awe of his unique state, leading to various conflicts.
It could be Shukaku. Since Shukaku was one of the first bijuu introduced in the series and has a sand - based power, it would be an interesting addition to Naruto's arsenal. Naruto's positive and energetic personality could also have an impact on Shukaku's usually grumpy and solitary nature.
Well, having two bijuu would likely make Naruto extremely powerful. He could potentially combine their unique chakra natures and abilities in combat. For example, if one bijuu has a fire - based ability and the other has a water - based one, he might be able to create new and unexpected jutsu by mixing them. Also, it would change his standing in the ninja world. People would probably be more in awe or more fearful of him, depending on their stance towards Naruto.
Naruto might be initially shocked. He has always seen the bijuu as powerful entities, not as potential mates. But being Naruto, he would likely approach the situation with his usual kindness and try to understand their feelings.
Naruto could face several challenges. Firstly, the mental strain of controlling such powerful entities. The bijuu have their own consciousness and desires, and keeping them in check mentally can be exhausting. Secondly, there's the issue of mastering different forms and abilities of the bijuu. Each bijuu has unique powers, and learning to utilize them effectively without losing control is no easy feat. Moreover, he may face social challenges as some in the ninja world may not trust him with this much power and try to undermine his control.
The power dynamics could be complex. Since Naruto is half - bijuu, he might have some powers that are a blend of human and bijuu abilities. The fem bijuu, being a full - fledged bijuu, would likely have more raw power initially. But Naruto could have the advantage of being able to think more like a human and strategize better.