Hi Kuldeep,
It doesn't appear that you can replace the rear cameras individually; rather it looks like they come as an assembly of all three and get replaced as a single unit. You have to remove the motherboard to get to them, but on your phone it's actually not too bad of a job. Sorry to say that iFixit doesn't have a guide for you, but there's a Spanish site called Nadie Me Llama Gallina that has exactly what you need.
[link|https://www.nadiemellamagallina.com/en/manuales/smartphones-1/oneplus-655/oneplus-7t-1159/c-maras-traseras-10287|Manuales / OnePlus 7T / Rear cameras|new_window=true]
The cameras themselves seem to be fairly easy to come by; take a look on places like Amazon, eBay and AliExpress for starters; you can undoubtedly find them in several other places as well. Here's an example of one of the first ones that came. up on my cursory search.
[link|https://www.aliexpress.us/item/3256802459638090.html|Original Front Back Camera For OnePlus 7 Pro One Plus 7T Backside Frontal Facing Rear Selfie Camera Module Repair Spare Parts - AliExpress|new_window=true]
Good luck; let us know how it goes!