How to calculate R,L,C on a bandpass filter?
Yes a band pass filter formed by a parallel L and C and fed via the resistor sounds the right approach but what you have to confirm is that the signal (1kHz square wave with 25% duty cycle) contains harmonic content at 3 kHz so, I'd advise you to do some Fourier analysis to confirm this.

Once you are happy that the original signal does contain 3kHz content then L and C are calculated like this: -

f = \$\dfrac{1}{2\pi\sqrt{LC}}\$.

The f term is frequency and in your case it is 3 kHz. This web tool might also prove useful: -