Is this what you want?
CREATE TABLE C
(
AFiled_a INTEGER NOT NULL,
AFiled_b INTEGER NOT NULL,
AFiled_c INTEGER NOT NULL,
BFiled_a INTEGER NOT NULL,
PRIMARY KEY (AFiled_a, AFiled_b, AFiled_c, BFiled_a),
KEY (AFiled_a, AFiled_b, AFiled_c),
KEY (BFiled_a)
)
;


CREATE TABLE B
(
Filed_a INTEGER NOT NULL,
BContent TEXT,
PRIMARY KEY (Filed_a)...