The rates of addition of HBr (dark) to the alkenes follow the order

The slow step is protonation to a carbocation. 2-Methylpropene gives a tertiary cation, propene a secondary one, and ethene the unstable primary ethyl cation (in the transition state). The rate order is therefore $(\mathrm{CH}{}_3){}_2\mathrm{C}=\mathrm{CH}{}_2>\mathrm{CH}{}_3\mathrm{CH}=\mathrm{CH}{}_2>\mathrm{CH}{}_2=\mathrm{CH}{}_2$.