To calculate the overall percentage gain or loss for each item, we need to apply the percentage increase and the successive discounts to each item's price, and then apply the sales tax. Let's do this step by step for each item.

Let's assume the original price of each item is $100 for simplicity. This will not affect the overall percentage calculation.

**Item A:**
1. Price increase by 31.5%: $100 + (31.5% of $100) = $100 + $31.5 = $131.5
2. First discount of 12.5%: $131.5 - (12.5% of $131.5) = $131.5 - $16.4375 = $115.0625
3. Second discount of 17.3%: $115.0625 - (17.3% of $115.0625) = $115.0625 - $19.9163 = $95.1462
4. Sales tax of 7.5%: $95.1462 + (7.5% of $95.1462) = $95.1462 + $7.1359 = $102.2821

Overall percentage gain/loss for item A: (($102.2821 - $100) / $100) * 100% = (2.2821 / 100) * 100% = 2.2821%

**Item B:**
1. Price increase by 27.8%: $100 + (27.8% of $100) = $100 + $27.8 = $127.8
2. First discount of 9.6%: $127.8 - (9.6% of $127.8) = $127.8 - $12.2688 = $115.5312
3. Second discount of 14.7%: $115.5312 - (14.7% of $115.5312) = $115.5312 - $16.9831 = $98.5481
4. Sales tax of 7.5%: $98.5481 + (7.5% of $98.5481) = $98.5481 + $7.3909 = $105.939

Overall percentage gain/loss for item B: (($105.939 - $100) / $100) * 100% = (5.939 / 100) * 100% = 5.939%

**Item C:**
1. Price increase by 33.1%: $100 + (33.1% of $100) = $100 + $33.1 = $133.1
2. First discount of 11.7%: $133.1 - (11.7% of $133.1) = $133.1 - $15.5727 = $117.5273
3. Second discount of 13.9%: $117.5273 - (13.9% of $117.5273) = $117.5273 - $16.3463 = $101.181
4. Sales tax of 7.5%: $101.181 + (7.5% of $101.181) = $101.181 + $7.5886 = $108.7696

Overall percentage gain/loss for item C: (($108.7696 - $100) / $100) * 100% = (8.7696 / 100) * 100% = 8.7696%

In conclusion, the overall percentage gain for each item after all the price changes and sales tax is:
- Item A: 2.2821% gain
- Item B: 5.939% gain
- Item C: 8.7696% gain
각 항목의 전체 백분율 증가 또는 손실을 계산하려면 각 항목의 가격에 백분율 증가와 연속 할인을 적용한 다음 판매세를 적용해야 합니다. 각 항목에 대해 단계별로 수행해 보겠습니다.

간단히 하기 위해 각 항목의 원래 가격이 $100이라고 가정해 보겠습니다. 이것은 전체 백분율 계산에는 영향을 미치지 않습니다.

**항목 A:**
1. 가격 인상 31.5%: $100 + (100의 31.5%) = $100 + $31.5 = $131.5
2. 첫 번째 할인 12.5%: $131.5 - (131.5의 12.5%) = $131.5 - $16.4375 = $115.0625
3. 두 번째 할인 17.3%: $115.0625 - (115.0625의 17.3%) = $115.0625 - $19.9163 = $95.1462
4. 판매세 7.5%: $95.1462 + (95.1462의 7.5%) = $95.1462 + $7.1359 = $102.2821

아이템 A의 전체 백분율 증가/손실: (($102.2821 - $100) / $100) * 100% = (2.2821 / 100) * 100% = 2.2821%

**항목 B:**
1. 가격 인상 27.8%: $100 + (100의 27.8%) = $100 + $27.8 = $127.8
2. 첫 번째 할인 9.6%: $127.8 - (127.8의 9.6%) = $127.8 - $12.2688 = $115.5312
3. 두 번째 할인 14.7%: $115.5312 - (115.5312의 14.7%) = $115.5312 - $16.9831 = $98.5481
4. 판매세 7.5%: $98.5481 + (98.5481의 7.5%) = $98.5481 + $7.3909 = $105.939

아이템 B의 전체 백분율 증가/손실: (($105.939 - $100) / $100) * 100% = (5.939 / 100) * 100% = 5.939%

**항목 C:**
1. 가격 인상 33.1%: $100 + (100의 33.1%) = $100 + $33.1 = $133.1
2. 첫 번째 할인 11.7%: $133.1 - (133.1의 11.7%) = $133.1 - $15.5727 = $117.5273
3. 두 번째 할인 13.9%: $117.5273 - (117.5273의 13.9%) = $117.5273 - $16.3463 = $101.181
4. 판매세 7.5%: $101.181 + (101.181의 7.5%) = $101.181 + $7.5886 = $108.7696

아이템 C의 전체 백분율 증가/손실: (($108.7696 - $100) / $100) * 100% = (8.7696 / 100) * 100% = 8.7696%

결론적으로 모든 가격 변동과 판매세를 적용한 각 항목의 전체 백분율 증가율은 다음과 같습니다:
- 항목 A: 2.2821% 증가
- 항목 B: 5.939% 증가
- 항목 C: 8.7696% 증가