Excel conditional formating

I need a "simple" formula & conditional formatting adding to a spreadsheet to either turn a line green or red depending on whether a task was completed within a set SLA priority timeframe which could either be A: 12 hours, B: 24 hours or C: 48 hous. The formula will need to look at 3 cells, startdate/time, finishdate/time & priority to establish whether line should be green or red.