I think in your scenario that B and C would drop their margin percentage because the tax does not represent anything that will increase their costs in handling the product. E.g. B would sell to C for $30 and C would sell retail for $50, making the same profit in dollar terms. If they don't, a competitor will, because a consumer will prefer the $50 price to the $80 price.
You're suggesting businesses B and C would just absorb the tariff cost by reducing their margin percentage to keep the same dollar profit. However, as Cerium noted earlier, many business costs do tend to scale with the price of goods, making this difficult without impacting profitability.
For financial costs, higher inventory value due to tariffs increases the cost of capital (interest on loans/tied-up funds), insurance premiums, and potentially transaction fees.
For operations costs (handling and labor), increases due to indirect effects: As the price of the average consumer basket increases due to tariffs, the average citizen either demands and receives higher wages or has to reduce spending; this means that the per-item variable cost of processing goes up, either because the wages of those employees increased (higher wages) or because the sales volume decreases (reduced spending).
Of course this does not mean that every business will have exactly this outcome. And absolute size of these effects is also dependent on the actual demand elasticity.
