I would just replace the doors and not the whole jamb. Confirm that the ACTUAL size of the doors you order will match your openings. Measure down from the top of your existing doors to figure out how far down your door handle hole is. Confirm that the new doors have the hole cut in the same place, or have them come with no hole and cut it yourself with a hole saw. You might measure down from the door top to the hinge mortises too to see if you can use their standard measurements to have them premortise the hinges for you, or do it yourself. It sounds like the doors you are ordering could be significantly more heavy than your existing hollow ones, plus they are 3' so they are large. This means you need three hinges, instead of the 2 that are sometimes used with hollow cores. Also you definitely need to use at least one long, strong screw in your top hinge, into the framing, to support the weight, as the original jambs might not be strong enough, in and of themselves to support the increased weight.