Every 30 seconds is to cut a video 3s
Solution 1:
The select filter is the most convenient for this,
ffmpeg -i in -vf "select='lt(mod(t,33),30)',setpts=N/FRAME_RATE/TB" -af "aselect='lt(mod(t,33),30)',asetpts=N/SR/TB" out
The video and audio select filters keep the first 30 seconds out of every 33 seconds. If your file doesn't have audio, drop the -af .. option.