3x√5 and 3(³√5x) are like radicals because they both have the same radicand (the number or expression inside the radical symbol) of 5 and the same index (the number that indicates the root being taken) of 3. The only difference is the coefficient (the number multiplying the radical) of 3 in front of the radical symbol, which is a factor that does not affect the radical itself.
x(³√5) and y √√5 are not like radicals to 3x√5 because they have different radicands.
√5y is not a like radical to 3x√5 because it has a different index.