A 36 door has a rough opening width of 38 so your header is cut at 41.
Window and door header framing.
Once you know the depth of the header it s width is cut to fit from king stud to king stud with the jack studs fitting under it to help disperse the loads.
The king studs nailed into the end of each header and the trimmers that butt just under the header combine to create a sturdy vertical support column.
Together the headers king studs and trimmers act as a system that transfers weight from above down and around the window and door openings to the floor and foundation below.
A typical header width with single jack studs is cut 3 larger than the rough opening.
Make a header by fitting two 2 x 8s around a 1 2 inch piece of plywood.
Install a cripple stud between the header and the top plate halfway between the king studs.
Not only would the structure be unsafe doors and windows over a period of time would not function properly.
Toenail the cripple stud to the top plate and the side of the header.
Cut the header to fit between the king studs.