I looked into learning how to conduct my own measurements, downloaded REW, watched a few youtube videos, visited some forums, etc., and quickly realized once again -- hell, no! Too much time and energy is required for me to learn it.
Instead, I invited a technically-astute, audiophile friend over my house to measure my low end room response using REW. Actually, it measured quite well, with the exception of a minor bump at 35Hz. We made a couple of attempts to remove the bump, but eventually, I was happy to leave the settings as is.
I finally sat down with a extended listening session last night and really like what the third sub is contributing. At times, it's subtle, but during bass heavy passages, it thumps hard! Love it. The vast majority of music I listen to is jazz vocals, so bass isn't a prominent feature of this genre until an upright bass is played or the drummer jams out. It's during these moments when the third sub provides a 3rd dimension to the bass, almost like an echo. It's akin to the difference between a drummer in a small jazz club vs. a lively concert hall. Hard to describe, but natural sounding nonetheless.