Installing three pre hung doors. 2 are interior doors and one is an exterior door. Past problems have been termites and wood rot. Would it be a good idea to paint the jambs with a wood preservative .What about the edges of the doors themselves? Exterior door is wood with a thin skin of steel over it.

Preservative only prevents rot... not termites...

You might want to saturate it with something the insects would not like... such as a liquid boric acid solution which will eventually dry and then use other treatments.

Perhaps someone will have a better idea than that...

Using flashing at the top of the door is key, as is trimming the outside with material that won't rot and won't let water in. I would use Great Stuff between the jamb and the studs and would calk around the trim with the best silicone 40 year calk. If the sill has a bevel where the trim will end, be sure to bevel the trim too.
(I'm in the north so maybe you don't need the insulation.)

Thanks a million. Kind of looks like a drip cap. Wonder if you can put them at the bottom of the door. Like the boric acid treatment too.

Yes, aluminum or vinyl edge under the door is good too, because water creeps back under and down the sill (the big one, the beam on the foundation) and foundation.
Boric acid can be dumped in there dry. But I'm not so sure it stops termites. The chemicals that get rid of termites are pretty strong stuff.

Rot doesn't just happen from water getting in; it also happens when standard moisture from indoors condenses upon hitting colder dryer air somewhere between the inside wall and the outdoors, often inside walls. That's why tar paper was replaced with Tyvek/Typar, which is breathable. Moisture along with warm air goes to the easiest outlets around windows and doors, and that's why there is the most rot there, even if the outside is well protected.

(Again I'm talking northern talk here.. )

Very Good stuff. Thanks