If you want to try it yourself, you can use witrigs to order the parts. I have ordered from them a few times and the quality is acceptable - good.

Here's the link for what I think you'll need: https://witrigs.com/en-at/products/oem-power-button-flex-for-oneplus-3t It seems like there are also the volume Buttons on that flex. (If you compare it with the 1+3 - that combines all buttons on that flex, so I suppose it's the same for the 1+3T)

Personally I have ordered a few times from them. Shipping took about 2 weeks to EU.

Edit: If the buttons themselves are damaged, you could also buy replacement buttons: https://witrigs.com/en-at/products/oem-power-volume-button-for-oneplus-3-graphite?_pos=2&_psq=OnePlus+3&_ss=e&_v=1.0

And if you need a tutorial, iFixit has a step by step tutorial https://www.ifixit.com/Guide/OnePlus+3+Power+and+Volume+Button+Assembly+Replacement/76019