The general case is that FIFO rules apply and that losses are offsettable against gains. There are two exceptions. One is regarding buying shares and selling them again within four weeks. The other is regarding selling shares at a loss (otherwise you can ignore this exception) and buying them again within four weeks.
1) If you buy shares and sell them again within four weeks of acquisition, FIFO rules do not apply -- the sold shares are presumed to be the ones you acquired most recently.
2) If you sell shares at a loss and reacquire shares of the same class with four weeks of the sale, the loss cannot be offset except against gains on the reacquired shares.
The answers to your questions based on this:
A) Yes, but if you sell share A three days after buying it, you are selling the ones you bought three days ago, not any other shares of class A that you might have held for longer. (i.e. FIFO rules do not apply because of the early sale).
B) You do not need to have held share B for 28 days in order to declare it as a loss. But if you have held it for less than 28 days it will be similar to your scenario A), i.e. FIFO rules do not apply. However, if -- after selling -- you then reacquire share B within four weeks, you can only offset the previous losses on share B with subsequent gains on share B.
C) correct.