If you use more than 16GB of RAM, meaning you have checked a monitoring utility or resource monitor, and are running low on memory during operations, then you MIGHT benefit from installing the other stick, and you might not even lose the dual channel operation from the two sticks you already have installed so long as THEY are in the A2 and B2 slots. But you MIGHT lose their dual channel operation which would be a hit to the memory bandwidth if you do.
I'm not sure there are any Ryzen systems that support any kind of FLEX mode operation or not honestly. That would be where the two matching sticks, installed in one channel (A2, B2) DO work in dual channel while the oddball stick operates only in single channel.
Otherwise, if you are not running low currently on memory due to large project resource requirements of the programs you run, then you'd probably be better off just sticking to the 16GB dual channel configuration you already have. If you can find another matching stick for your 16GB stick and pair it with the one you have now, and repurpose the other two sticks elsewhere or sell them, that might work better for you. Or even add it TO those sticks, but I'll tell you that anytime you start adding oddball or mixed memory together you start increasing the probability that one or more of them is not going to want to "play nice" with the others.
*Resolving memory problems and setting up XMP/DOCP/AMP profiles
As is, you probably want the two existing sticks in the A2 and B2 slots and the other stick in either of the other two slots. Doesn't really matter which of the other slots that stick is installed in. If it interferes with your CPU cooler being in the slot closest to the CPU, use the other open slot.