In order for you to play MXGP3 - The Official Motocross Videogame on Linux with minimal graphics settings, your computer must meet the following requirements
Operating System:
Ubuntu 16.04 or later, SteamOS 2.0
Processor (CPU):
Intel Core i5-2500K; AMD FX-6350 or equivalent
Memory (RAM):
4 GB RAM
Graphics Card:
NVIDIA GeForce GTX 760 with 2 GB VRAM or more; AMD Radeon HD 7950 with 2 GB VRAM or more
Storage:
13 GB available space