When the mining reward go down from 3 to 2 ETH it means a (3-2)/3=1/3=33% reduction. The price of a coin is generally correlated with the cost of mining. The cost of mining Ethereum goes up 1/(2/3)-1=3/2-1=1/2=50% so naturally the price should with time continually seek its way to 50% more than the balanced price level before the reduction.
The reduction of the mining reward with 1/3 should lead to a price increase of 50%.