Sorry, I may have misunderstood. I thought you meant doing the 3 hills in one long day.
if you are doing them individually, then that is much more straightforward. The school kids have an annual race up Mt Tumbledown which takes about 45-60 minutes from the west end of the harbour. Mt Longdon is a bit further to walk - probably about 2 miles from the road at the western end of the harbour, and another mile from the centre of town. Once you are on the ridge (follow the road to the top), its a fairly flat walk, and the summit is clearly visible. If you then headed south from there to Tumbledown or Harriet, you would have to cross miles of pathless peat ground, which you probably remember. I would suggest its probably easier retracing your steps.
Harriet is right by the main road, about 2-3 miles west of Stanley, and is a "straight up and down" walk of less than a mile. So, impossible to get lost, and easily recognised by the memorials in the large car park at the side of the road. So, again, much simpler to do on its own than combining with other hills.
My apologies for earlier response, but it's not unknown for people to assume, because the distances aren't that great, that its a fairly easy day out. But, as you probably know, the underfoot conditions can be energy-sapping. Hope you enjoy your trip, and if you have more questions, I'll try to respond, Peter