1 2 3
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1) + + +
2) + + -
3) + - +
4) - + +
5) - - +
6) - + -
7) + - -
8) - - -
The 8 outcomes are equally likely, so the probability of each is 1/8.
In 4 of the 8 possible outcomes, 2 or more of the stocks increase in value. So the probability that at least 2 stocks increase in value is 4/8 = 1/2.
Note that result can be obtained using logical reasoning: for each case where 2 or 3 stocks increase in value, there is an "opposite" case in which 0 or 1 stocks increase in value. So in exactly half of the cases the number of stocks that increase in value is 2 or more.