For each coin, there are 2 outcomes, heads (H) or tails (T), so for the 3 coins, the number of outcomes are:
2 x 2 x 2 = 8 outcomes:
HHH / HHT / HTH / THH / TTT / TTH / THT / HTT
Out of all these outcomes, there are 3 ways, we get exactly 2 Heads:
HHT / HTH / THH
So, the probability of exactly 2 coins landing on Heads is 3/8, or 0.375