1/4 isn't correct because it's more likely you'll have, say, 2 boys and 1 girl than it is that you'll have 3 boys. One (long, but easily manageable in two minutes) way to do the problem is to list all of the possible sequences of boys and girls:

BBB
BBG
BGB
GBB
BGG
GBG
GGB
GGG

There are 8 sequences, and in 3 of them we have exactly two girls, so the answer is 3/8.

There are faster ways to do these problems, which have been posted in this forum before, so I won't repeat them, but you might search for Stuart Kovinsky's post from a couple of years ago called something like "coin flip problems made easy".