They can all produce false positives (PSMA-PET most of all, I think), so using more than one type of scan can help make them more confident about separating the signal from the noise. E.g. if something shows up on all three, then there's a stronger chance it's cancer-involved than if it shows up on just one.

Here's an example with two scans. I had a metastasis on my spine in 2021, which was treated with emergency debulking surgery and then radiation. My most recent CT scan last spring showed some lucency there, but a bone scan (which would be more accurate for the spine) showed nothing. Combine that with the facts that my ALP blood test is in normal range (it would often be elevated with a new bone metastasis), that my PSA remains undetectable (< 0.01) on the ultrasensitive test, and that it's exactly where the surgeon operated and fused my spine with cement and screws 4 years ago, and both the radiologist and oncologist are highly confident that the CT lucency is just showing my bone healing, not anything cancer-related. (Still, we plan an MRI next week just to be extra, extra certain.)

That's the way it goes. No one test is decisive, but the more positives you get, the more "votes" you have that there might be cancer.