This page links to where you can acquire useful art tools to produce the content you need for your games.
A free modeling program that works on multiple platforms. Can be downloaded from their site, here.
Image editing programs
A free image editing program often used as an alternative to photoshop. You can get it from their site, here.
These are tools designed to aid in the production of Physically Based Render-compatible materials.
A suite of tools that can convert images to have the various PBR maps, design a new material from the ground up, and use materials to directly 3d paint PBR materials onto a model.
For indies, you can acquire a rent-to-own license at $20 a month or pay a flat license fee at their site, here.
Because T3D's PBR materials utilize a composite map to keep memory and performance as good as possible, if you hand it separate metal, roughness and ao maps, it will compositize it internally, or you can produce a composite map in your tools to make loading the materials faster.
Here is a substance designer file that will let you plug in the various maps and output a composite map to save some time and effort.