For a channel of 3 KHz bandwidth and signal to noise ratio of 30 dB, the maximum data rate is :

By Shannon's theorem the maximum data rate of a noisy channel is $C = B\log_2(1 + S/N)$. A signal-to-noise ratio of 30 dB means $S/N = 10^{30/10} = 1000$, so $C = 3000 \times \log_2 1001 \approx 3000 \times 9.97 \approx 29{,}900$ bps, or about 30,000 bps. The figure of 6000 bps is the Nyquist rate $2B$ for a noiseless channel with only two signal levels, which leaves the noise out of account.