Nucleophilicity of alcoxy groups
$$\Large\color{red}{\text{nucleophilicity} \ne \text{basicity}}$$

Please memorise firmly that nucleophilicity and basicity, albeit related in many ways, are different physico-chemical properties. Most importantly, basicity is a thermodynamic property while nucleophilicity is a kinetic property. Therefore, it is not uncommon for compound $\ce{A}$ to be more basic but less nucleophilic than compound $\ce{B}$.

This is the case here, too. It is unquestioned and supported by the corresponding $\mathrm{p}K_\mathrm{a}$ that tert-butanolate is the strongest base, followed by isopropanolate followed by ethanolate. This is due to the $+I$ effects you correctly outlined.

Nucleophilicity does not parallel this series since the it does not abide by the same rules. For a compound to be nucleophilic, the nucleophilic atom’s lone pairs must be able to actually attack, i.e. reach, the electrophilic site it wishes to attack. This means two molecules interacting and not just one molecule interacting with a proton which is so small it can even tunnel. In tert-butanolate, you have three &#x3b1;-methyl groups that take up space and make said interaction harder. Due to this steric interaction, tert-butanolate is an absolutely lousy nucleophile (while it is a great base).

It is similar for isopropanolate which still has too much steric hindrance to be an effective nucleophile although the conditions are better than for tert-butanolate.