How to make only 3 biomes appear in overworld

This isn't possible without rewriting a large portion of Minecraft's code.

• You would have to rewrite most of minecrafts world gen code
  • If you don't know Java/JSON at an advanced level, you won't be able to do this
• Recommended Alternative
  • Make the three biomes as custom biomes
    • Mimic/Copy Forest
    • Mimic/Copy Desert
    • Mimic/Copy Plains
  • Make all three biomes use 1/3 of the temperature setting and take up all of the other settings
    • Forest: -1 to -0.33
    • Plains: -0.33 to 0.33
    • Desert: 0.33 to 1
  • Set up features, structure, etc that you want to generate in those biomes
  • Set up entity spawns to mimic the existing entity spawns
  • Issues with this option:
    • Still a small chance other biomes will spawn
    • There will still be mountains and oceans, just painted with your custom biomes
• Other Solution
  • Make a custom dimension with just these three biomes
  • Force the player to be teleported to that dimension when they spawn into they spawn into and/or enter the overworld at any point.
  • There will be issues with this solution
    • You'll be set in a random location every time you travel in and out of the nether
    • Won't be able to make nether travel networks
    • Coul try making a location variable to save the players location in the world and teleport them to that location when they leave other dimensions but you will also have to make a portal spawn where the player gets sent if there isn't already a portal there

I do not/cannot offer any other solutions.