Welcome Zac!
Glad to hear you will try discus at last! Looks like you have done your research well to start. The larger the discus the less likely they will be problematic if they are healthy.
Would you consider holding off on any plants or substrate for about 6 months? If so... You could easily grow your 3 inch option to 4-5" in that 125 gal. I would suggest getting 12-15 if you really want a good 10-12 adults as you may lose a few along the way while you are learning.
Al