Exactly 3 of 7 circles here should be painted black to make the Masyu uniquely solved. Find them!
I've found one solution, through some logical reasoning followed by trial and error.
Ignore the bottom left figure
I first figured that the circle in position [2, 1] (Red) must be white, because if it were black, circle in position [1, 3] (Green) would not be able to connect. Then for similar reasons, the yellow and cyan ones can not both be black. The same is true for cyan and purple. A few more similar deductions, like blue, purple, and black can not all be black, left me with eight possible arrangements, so I just tried to solve them all. Only one of them has a solution, and the process of solving it showed that it is unique (I don't know how to prove that though)
Thanks to Skynet_0 for pointing out that one of my solutions is invalid.