PSP, CBD and MSA can only be clinically diagnosed and confirmed by only an autopsy. There are some scans that help doctors better determine which they think it is but not many are specialized in this. They suspected PSP for my mom and mom was given the official clinical diagnosis in late 2018 and it was confirmed PSP on her autopsy in May 2019. There are also some symptoms that can help distinguish them. (PSP is known for the eye gaze and issues)
Here is a link to CurePSP where they describe the three in detail.
They thought my mom may have MSA due to some symptoms that presented more as MSA than PSP, like urinary incontinence. Unfortunately they told us that it didn’t particularly matter in regards of treatment and care, as they all follow a similar path and care need. It does however matter for research purposes and to try and better understand these diseases. (it is so important to have a correct diagnosis confirmed, which can only be done by autopsy.)
I found it very confusing as I thought my mom showed more MSA signs than PSP but as it progressed the PSP hallmark symptom came....The eyes....my mom started losing the ability to move her eyes and focus properly.
Sending you strength and love