Either way, (in my opinion) if you can't provide standard IFR separation or reduced separation between the two flights, you probably shouldn't be launching the departure.
5 'n' 3 - that would mean that you can only independently use two runways if they are located 5nm apart (or 10nm if you are in a region where 10nm is stipulated as the required radar separation), unless there is a superhuman way of controlling them by the same controller whereby you could reduce the distance to 3nm. I'm sure that would be fine at Heathrow, and still room for a 3rd.......