Not that I'm bragging, but I've never gotten higher than a 3 in Home Economics on my report card. On a ten-point scale.
Literal
Brag is-not-[though-が] I [topic-は] report-card [gen-の] home-economics [at-で] 3 or-above [obj-を] took [thing-こと] don't-exist. 10-point evaluation [at-で].
自慢じゃないが is a set discourse opener for ironic self-deprecation — 'not to brag, but...' which typically precedes something embarrassing rather than boasting. 通知表 ('report card') is the traditional Japanese school grade report. 家庭科 ('home economics') is the subject taught in Japanese schools covering cooking, sewing, and domestic skills. ~たことがない ('have never X-ed') is the experience-negative pattern, with the added ~が expressing 'no instances of X.' 10段階評価 ('ten-level evaluation') is a Japanese grading scale. The delayed specification at the end ('10-point scale') punctuates the humor.