$<1^{\circ}<2^{\circ}<3^{\circ}$

Given carbocations $(\mathrm{CH} _3) _3 \mathrm{C} \cdot \Rightarrow 3^{\circ}$ carbocation

$(\mathrm{CH} _3) _2 \mathrm{CH} \cdot=>2^{\circ}$ carbocation

$\mathrm{CH} _3 \mathrm{CH} _2 \cdot \Rightarrow 1^{\circ}$ carbocation

Therefore, increasing order of stability of carbocations $\mathrm{CH} _3 \mathrm{CH}_ 2 \cdot\left(1^{\circ}\right)<(\mathrm{CH}_ 3) _2 \mathrm{CH} \cdot\left(2^{\circ}\right)<(\mathrm{CH}_3)_ 3 \mathrm{C} \cdot\left(3^{\circ}\right)$