How to remove "Show All" from Home Page

Hello,

Need some help regarding this: My posts are in three main categories: A, B, & C. The categories are completely unrelated to each other. So, I do NOT want the “Show All” filter on Home Screen where it shows posts from all categories together. Instead, I want the default filter to be on Category A only when page opens first. So, I just want posts to show from either Category A, B or C but not all together as in “Show All”. What I want is the “FILTER BY” on home screen to only show Category A, B and C with default selection on Category A.

I’d really appreciate if someone can help me find out what code changes need to be done to accomplish this.