What is the minimum requirements for a TS3 server?

For Windows:
Windows 8.1 and newer, 32 and 64-bit

For Mac OS X:
Mac OS 10.14 Mojave and above, 32 and 64-bit Intel CPU

For Linux:
Any recent Linux distribution, 32 and 64-bit

For FreeBSD:
10.2 and above, 32 and 64-bit

For the CPU:
SSE2 capable CPU
SSE4.x capable CPU with POPCNT enabled (Only for the 64-bit version on Windows)

For Memory:
This depends on the operating system being used, however a minimum of 1 GB RAM is recommended.
Total memory usage depends on the number and size of hosted virtual servers.

For the Hard disk:
The bare minimum installation is 15 MB.
Total disk usage is highly dependent on actual usage, size and the amount of hosted virtual servers.
Additional space is required for file transfer and for icon/avatar storage.