I've found 6 x 5 x 1 @ 105-108% of FTP to be effective intervals. The first couple feel relatively easy and the last couple are a struggle.

The 8 x 3 x 1 intervals you mentioned are likely your favorite because they're easy. If you took out the 1 min rest periods you could have 32 min of what is commonly referred to as Sweet Spot Training (SST) which I also find to be effective and something you can do for a few days in a row.