What is the speed of the train is kmph?
Statement: I. The train crosses an ‘x’ metre long platform in ‘n’ seconds.
II. Length of the train is ‘y’ metres.
III. The train crosses a single pole in ‘m’ seconds.

Clearly, each statement alone is insufficient.

Let speed of the train = s kmph

I & II : Length of train = y, length of platform = x, time taken = n

=> $$\frac{x + y}{s}$$ = n

=> s = $$\frac{n}{x + y}$$ [ where, x,y,n are constants.]

Thus, I & II are sufficient.

II & III : Length of train = y, time taken = m

=> s = y/m

Thus, II & III are sufficient.

I & III : Let length of train = L

=> $$\frac{L}{s}$$ = m [From III]

& $$\frac{x + L}{s}$$ = n [From I]

Since, we have two variables 'L' & 's' and two equations. Therefore, we can find the speed of train.

Thus, I & III are sufficient.

Thus, any two of the three statements are sufficient.