You only want to use 80% of your PSU. So having an 850 watt PSU, you want to run 680 watts max off it. Remember, that includes motherboard, CPU, etc. as most mining software only give you the wattage of the GPU. Getting a Kill-a-Watt or something similar will help determine your true power draw.
Looking at whattomine for a base, it says the 3080 TI runs at 280 watts on Autolykos. Three cards would be 840 watts which is to much for the PSU. Depending on the rest of your hardware wattage, you could likely run two off that PSU.
You can likely undervolt, but be careful as sometimes the default settings come back when a computer freezes when mining. For clarity, if you were able to undervolt to 200 watts per card, you could run all three. However, if the default settings came back and you were at 280 per card, you’re going to overload something. Numbers are hypothetical, I don’t have a 3080 TI to test any undervolts against.
Remember, you’re going to be generating heat so these are guidelines. If you’re somewhere really hot, you will want to watch temperatures as an 80% used PSU could still have issues. Be careful and make sure you’re doing everything safely for what works in your environment.