I would recommend 'Ace, Luffy and Sabo: Reunion of Souls'. This fanfiction is full of heart - warming moments. It shows how their past experiences shape their present interactions and how they support each other in the face of difficulties.
There's a fanfic called 'Sabo, Luffy and Ace: Unbreakable'. In this fanfic, it explores their past, present and future. It shows how their shared experiences have shaped them. It also has some interesting plot twists that keep you hooked from start to finish.
Well, there are fanfictions that explore their emotional bonds. For example, a story where Luffy gets seriously injured during a fight and Ace and Sabo have to work together to save him. This would not only show their physical strength but also their deep love and loyalty for each other as brothers. And some fanfictions might also involve them facing a powerful new enemy together that forces them to rely on their unique skills and the power of their brotherhood.
Rediscovery of their relationship after separation can be a theme as well. Given their backstories in One Piece where they were separated at times, in the fanfictions, their reunion can be the starting point of a new, more romantic relationship.
In Ace x Luffy x Sabo fanfics, the sense of loyalty among them is a key element. It's shown in so many ways, like when they protect each other from danger. The fanfics also often explore their dreams and how they support each other to achieve them. The relationships they have with other characters in the One Piece universe are also interestingly developed in these fanfics, which adds more depth to the story.
Another way is to follow some One Piece fanfiction writers on social media platforms. They often share their works or recommend other great fanfictions. Also, if you know any other One Piece fans, ask them for suggestions. They might know some hidden gems. Some fanfiction communities on Reddit can also be a good source. You can search for One Piece fanfiction subreddits and then specifically look for luffy x ace x sabo stories there.
Archive of Our Own (AO3) is a great place to start. It has a large collection of fanfictions, including many for 'ace x luffy x sabo'. The search function there is quite effective in helping you find the ones that suit your taste.
Fanfiction.net might also be a good place. However, the tagging system there might not be as detailed as AO3. But still, it's worth a try. Look for sections dedicated to One Piece and then search specifically for stories involving Ace, Luffy and Sabo in a yaoi context.
Typically, Sabo x Luffy fanfiction emphasizes their strong brotherly bond. It often showcases their loyalty to each other. Many stories also explore their shared dreams and how they support one another in achieving those dreams.
One great 'Ace Sabo Luffy Fanfiction' is 'The Brothers' Bond'. It beautifully explores their relationship, showing how they support each other through various challenges. Another one is 'Sabo's Return'. It focuses on Sabo coming back into Luffy and Ace's lives and how they all readjust. There's also 'Ace and Luffy's Adventure', which takes the three on a fictional journey filled with action and heartwarming moments.