You can use any 3D program to create content as long as you can export them out as a filetype that 3DSmax can recognize (.obj, .3ds, .fbz etc)
3DS Max is currently the only program able to export models (using NiNtoxicated01's export script, and later when Blizzard releases their art tools). You can download the trial for 3DS Max for free, and it lasts 30 days. It's usable to export using the max scripts, and fully functional for that duration.
You can use any free program to model/texture custom content, then simply install 3DS Max trial when you're ready to export. The only other solution is Gmax; though I don't know if the exporter is compatible with that.