Here is Assassin’s Creed 3 Minimum PC System Requirements:
Supported OS: Windows Vista (SP2) / Windows 7 (SP1) / Windows 8
Processor: 2.66 GHz Intel Core2 Duo E6700 or 3.00 GHz AMD Athlon 64 X2 6000+ or better recommended
RAM: 2 GB (4 GB recommended)
Video Card: 512 MB DirectX 9.0c-compliant with Shader Model 4.0 or higher (see supported list)*
Sound Card: DirectX 9.0c-compliant (5.1 surround sound recommended)
DVD-ROM: Dual-layer drive
Hard Drive Space: 17 GB
Peripherals Supported: Windows-compatible keyboard, mouse, optional controller (Xbox 360 Controller for Windows recommended)
Multiplayer: 256 kbps or faster broadband connection
Supported Video Cards at Time of Release:
AMD Radeon HD 3870 / 4000 / 5000 / 6000 / 7000 series or better
NVIDIA GeForce 8600 GT / 9 / 100 / 200 / 300 / 400 / 500 / 600 series or better