<p>Trees have been proven to be an effective measure for improving thermal comfort in urban street canyons. Crown height (<i>CH</i>) and tree coverage ratio (<i>TCR</i>) are key factors for shading and cooling efficiency. However, the combined effect of these factors on PET from a three-dimensional perspective remains unclear. This study, using both field measurements and ENVI-met simulations, quantified the independent impacts of 102 tree planting strategies on pavement shading and PET. A three-dimensional greening index (<i>3DGI</i>) was further introduced to quantify the combined effects of these parameters. The results showed that the independent contribution of trees to shading primarily reduces radiative heat transfer, alleviating thermal discomfort. The thermal environment on both sides of the street exhibited significant heterogeneity, with the greatest contribution to shading observed on the north side of E-W street canyons. Based on these findings, the study proposed optimized tree planting strategies for different street orientations and provided the suitable <i>3DGI</i> ranges for each street orientation, such as 536.30–548.95 for the east side of N-S street canyons. The study recommends prioritizing evergreen trees with greater <i>CH</i>, and adjusting <i>TCR</i> to ensure tree planting strategies on both sides are within the appropriate <i>3DGI</i> range. Future research will further expand the application of <i>3DGI</i>, integrating different climatic conditions and time scales to enhance urban residents’ thermal comfort and improve their quality of life.</p>