Most shoppers use one discount at a time: a coupon code, or a cashback app, or a card reward, never all three together. That's leaving money on the table, because most retailers don't actually block you from combining them. They just don't advertise that you can.
The order matters more than people realize. Start with the store's own coupon code at checkout first. That's the discount most likely to have restrictions on stacking, so applying it first lets you see immediately if it conflicts with anything else. If it goes through cleanly, move on to a cashback browser extension.
Cashback extensions work behind the scenes and rarely conflict with a coupon code, since the store coupon changes the price you pay, while cashback tracks a percentage of whatever that final price ends up being. That means the discount actually compounds instead of competing.
The last layer is a rewards credit card with an active category bonus. If your card offers 5% back on a category the store falls into that quarter, you get that on top of everything else, since the card issuer has no visibility into what coupon or cashback deal you already used.
The one thing to watch for: some stores explicitly say 'one discount per order' in their terms. When that's the case, stacking a coupon with a cashback extension usually still works because the store never sees the cashback app as a 'discount' on their end, it's tracked separately by the extension. It's really only stacking two of the store's own codes that gets blocked.
Try this the next time you check out anywhere: coupon code first, cashback extension active in the background, and a rewards card with a live category bonus. Small stack, real savings.