A shape forming method for fabricating a fibrous preform with three-dimensional configuration, comprising: (a) providing a tool housing with a mold positioned therein, said mold having a non-planar molding surface; (b) placing a first moldable sub-preform over the molding surface; (c) attaching a first flexible diaphragm to the tool housing so as to define a sealed chamber bounded by the first diaphragm and the tool housing and to enclose the first sub-preform and the mold in said sealed chamber; (d) enclosing a second moldable sub-preform between the first diaphragm and a second flexible diaphragm, which is placed above the first diaphragm, the first and second diaphragms (12, 13) defining an air-tight sealed pocket; (e) applying heat to the sub-preforms (S1, S2); (f) evacuating air from the sealed chamber bound by the first diaphragm and the tool housing until a vacuum pressure is reached such that the first diaphragm is pulled toward the molding surface, causing the first sub-preform to conform to the molding surface, thereby forming a first shaped sub-preform, and concurrently with evacuating air from the sealed chamber, venting the sealed pocket between the first and second diaphragms (12, 13) to atmospheric pressure to prevent the second diaphragm from being pulled toward the molding surface at the same time; (g) evacuating air from the sealed pocket between the first and second diaphragms (12, 13) until a vacuum pressure is reached such that the second diaphragm is pulled toward the mold, causing the second sub-preform to conform to the outer surface of the first sub-preform, thereby forming a second shaped sub-preform; (h) cooling the shaped sub-preforms; (h') upon cooling of the sub-preforms, sequentially relieving the vacuum between the diaphragms (12, 13, 14) and between diaphragm and the tool housing by venting to atmosphere and removing the shaped sub-preforms sequentially; and (i) assembling the shaped sub-preforms into a final preform that can be infused with a liquid resin, wherein each sub-preform (S1, S2) comprises a plurality of fibrous layers bonded to each other and each sub-preform (S1, S2) is permeable to liquid resin. (a) providing a tool housing with a mold positioned therein, said mold having a non-planar molding surface; (b) placing a first moldable sub-preform over the molding surface; (c) attaching a first flexible diaphragm to the tool housing so as to define a sealed chamber bounded by the first diaphragm and the tool housing and to enclose the first sub-preform and the mold in said sealed chamber; (d) enclosing a second moldable sub-preform between the first diaphragm and a second flexible diaphragm, which is placed above the first diaphragm, the first and second diaphragms (12, 13) defining an air-tight sealed pocket; (e) applying heat to the sub-preforms (S1, S2); (f) evacuating air from the sealed chamber bound by the first diaphragm and the tool housing until a vacuum pressure is reached such that the first diaphragm is pulled toward the molding surface, causing the first sub-preform to conform to the molding surface, thereby forming a first shaped sub-preform, and concurrently with evacuating air from the sealed chamber, venting the sealed pocket between the first and second diaphragms (12, 13) to atmospheric pressure to prevent the second diaphragm from being pulled toward the molding surface at the same time; (g) evacuating air from the sealed pocket between the first and second diaphragms (12, 13) until a vacuum pressure is reached such that the second diaphragm is pulled toward the mold, causing the second sub-preform to conform to the outer surface of the first sub-preform, thereby forming a second shaped sub-preform; (h) cooling the shaped sub-preforms; (h') upon cooling of the sub-preforms, sequentially relieving the vacuum between the diaphragms (12, 13, 14) and between diaphragm and the tool housing by venting to atmosphere and removing the shaped sub-preforms sequentially; and (i) assembling the shaped sub-preforms into a final preform that can be infused with a liquid resin,