Step 1: know what speeds up addition of HCN.
HCN adds to a carbonyl when its cyanide attacks the carbonyl carbon. Two things decide how easy this is. If the carbon is crowded by big groups, the attacking cyanide struggles to reach it, which slows the reaction. Also, alkyl groups push electrons toward the carbonyl carbon and make it less hungry for the nucleophile. So fewer and smaller groups means faster reaction.
Step 2: compare the three compounds.
$\mathrm{CH_3CHO}$ has just one small methyl plus a hydrogen on the carbonyl carbon, so it is the most open and the most reactive. $\mathrm{CH_3COCH_3}$ has two methyls, a bit more crowded, so it sits in the middle. $\mathrm{(CH_3)_3C\text{-}CO\text{-}CH_3}$ carries a bulky tert-butyl group beside the carbonyl, which blocks the approach badly, making it the slowest.
Step 3: write the increasing order.
From slowest to fastest: $\mathrm{(CH_3)_3C\text{-}CO\text{-}CH_3}$ < $\mathrm{CH_3COCH_3}$ < $\mathrm{CH_3CHO}$.
Final answer: $\mathrm{(CH_3)_3C\text{-}CO\text{-}CH_3}$ < $\mathrm{CH_3COCH_3}$ < $\mathrm{CH_3CHO}$.