Yes, there are quite a few. Some focus on their adventures together, imagining how their different personalities and abilities would interact. For example, a story might have them team up against a powerful new enemy in the One Piece world. They could combine Law's devil fruit powers, Kid's mechanical skills, and Luffy's stretching abilities in unique ways to defeat the foe.
You can start by looking on popular fanfiction websites like Archive of Our Own (AO3). It has a large collection of One Piece fanfictions, and you can easily search for Law x Kid x Luffy stories using the tags and search function. Another option is FanFiction.net, which also has a good number of such fanfictions.
Nami could be a main character. She would be very worried about Luffy and try to find him. She might use her navigation skills to track down Law's ship. And she could be the one to try and negotiate with Law for Luffy's release.
One great fanfiction could be 'The Unlikely Alliance' where Kid, Fem Luffy and Law team up against a new and powerful enemy in the New World. Their interactions are full of bickering at first but then they start to understand each other's motives.
In many fanfictions, the relationships start off rocky. For example, Kid and Law might not trust Fem Luffy's overly optimistic and sometimes reckless nature at first. But as they face challenges together, like fighting a strong pirate crew or dealing with a difficult situation on an island, they start to see her strengths. And Fem Luffy, with her big heart, gradually breaks down the walls that Kid and Law have built around themselves.
Luffy x Law fanfiction often explores unique relationships. In such works, the interactions between their distinct personalities can be really interesting. Luffy's carefree and bold nature contrasts with Law's more reserved and strategic side. The 'lemon' aspect might add some romantic or tense elements to their relationship, but it also depends on how the author portrays it.
One common theme is friendship. Fans like to explore how their different personalities can form a strong bond. Luffy's carefree and positive nature contrasts with Law's more serious and calculated demeanor, and seeing how they interact as friends is a popular theme.
In many Luffy fanfictions, a jealous Law can be quite an interesting character. He might be jealous of Luffy's popularity among the crew or his carefree attitude. This jealousy could lead to some tense moments between them, like when Law tries to outdo Luffy in certain tasks just to prove himself better.
I really like 'Luffy and Law's Shared Destiny'. It shows how their fates could be intertwined in a more romantic way. The story has some really sweet moments between them, like when they share their dreams under the starlit sky during a break in their journey. It also adds new elements to the One Piece world they exist in.