The carbocation is stabilized due to weak +I (inductive) effect of methyl groups (-CH₃)

This carbocation is directly linked with –OCH₃ group. Oxygen has 2 lone pairs of electrons so it readily donates a pair of electrons to stabilise the positive charge on carbon through +R effect(resonance by donating electrons) which is stronger than +I effect. Hence, this cation is highly stabilized.

In this case the –OCH₃ group is attached farther from carbocation so will exhibit a weak –I effect( pulling of electrons towards itself). Hence, it is the least stable cation out the three.