Can dermal fillers be used to lengthen the columella and infra-tip? Rhinoplasty done 3x previously, will this cause necrosis?

You are quite right to be cautious. However, an appropriately cohesive, low viscosity HA filler can be added to the columella and nasal tip regions. This is not a region for the hands of a novice injector; this should best be trusted to and performed by a very experienced filler injector. You would be wise to make sure to consult a board certified cosmetic dermatologist and best of luck.

Generally speaking, dermal fillers are not recommended at sites of prior invasive surgery especially in high risk areas with a variable blood supply. This is because the surgery can affect the normal vascular anatomy making the risk of vascular occlusion and its requisite complications higher.