I have two sealed 10 inch subs, a Sunfire TSEQ10 and a Klipsch R-10. You made a suggestion I didn't think of, replacing one of my existing subs. The Sunfire is like 1200 watts vs the Klipsch 300 but since they are both sealed 10 inchers they harmonize well.
The third sub doesn't have to be big, it is just to smooth out response at high volumes. I have like 14 speakers in my room in an Atmos setup I thought it would be like more even. I am thinking of getting the Sunfire HRS-8 (1000 watts) which is an 8 inch sealed sub and I could mount it high on the wall in a speaker bracket. I am using my living room as a media room so space is an issue.