Simulink does not support an ideal filter block. As a substitute, a non-ideal filter with a very small transition band can be created. A filter structure of "Direct-form FIR" with a passband just below 3000Hz and a stopband of just over 3000Hz should achieve this. Below shows one example of this filter.
Frequency response for the above filter:
Alternatively, you can design your own filter using the Analog Filter Design block. This won't be an ideal filter either, but may be more desirable for your implementation. To examine the frequency response of, for instance, a custom Butterworth filter, see https://www.mathworks.com/help/signal/ref/butter.html. Other options include cheby1, cheby2, ellip, and besself.
Further information on the pre-R2015b Lowpass Filter Simulink block can be found here: https://www.mathworks.com/help/dsp/ref/lowpassfilterobsolete.html .
