JPF wrote:What do you mean by single maximal 3-rookery?
999...... - 27 clues with all 9 digits of the three clues ..... and actually 999221110 non-minimal puzzles are quite common !
looking at any non minimal puzzle with 999221110 the ...221110 clues tend to be essential .
There are many ways of removing clues to give a minimal puzzle but the minimal 666221110 wasn't very common
And of course the 19 clues with 543221110 is 61