If the 3rd is Kentucky, let's look at a hypothetical 4-year schedule cycle:
2026: Alabama, at Vanderbilt, Kentucky, Texas, Missouri, Auburn, at Ole Miss, at South Carolina, at Arkansas
2027: at Alabama, Vanderbilt, at Kentucky, at Florida, Georgia, at Mississippi State, Oklahoma, at LSU, Texas A&M
2028: Alabama, at Vanderbilt, Kentucky, at Texas, at Missouri, at Auburn, Ole Miss, South Carolina, Arkansas
2029: at Alabama, Vanderbilt, at Kentucky, Florida, at Georgia, Mississippi State, at Oklahoma, LSU, at Texas A&M
I'm sure this isn't perfect, but it's an idea of what it might look like. We alternate 5 home SEC games one year with 4 the next.
This also shows that no matter who the 3rd permanent opponent is, the schedule will be tough. Add in the fact that we have Georgia Tech away and home in 2026/2027, West Virginia in Charlotte in 2028, and Washington home and away in 2029/2030, and these schedules will be plenty difficult.