but the amount of data  to be worked is more than it can handle"

If you can create, for each group, formulae in 3 cells, one each for
<,>, and =, each of which results in TRUE or FALSE (hopefully ending
up with only one TRUE one out of the three) then you should be able to
use the same formulae in the 'Formula Is' option instead of 'Cell
Value Is' in conditional  formatting of one cell. Then you'll be able
to delete the formulae in the 3 cells.