I don’t regret it.
So, unless the first two opinions are completely identical - yeah, definitely get the third. Better to go for people with slightly different specializations if relevant.
In the first case, I decided to talk to an expert in the particular area who then ordered different tests. These indicated the issue had actually gone away completely, and I avoided surgery as a result of those.
In the other, a grade 2 ACL tear, it was less clear-cut, but I felt like the explanations given by the second doctor as to why he'd recommend against surgery were clearer and made more sense to me.
I'm glad I got both second opinions and the first case in particular, made me much more likely to get second opinions in future as well as more confident about doing so.