Question

how can we recognise the planets venus, Mars, and saturn in the sky

Answer 1

Venus is easy: it's the white star seen in the west after sundown or east before sunrise that's not part of a familiar constellation. It's particularly easy when it is far brighter than any othe star in the sky. (It can be easily confused with Mercury when it is dim.) Mars and Saturn can be tricky. Both appear reddish, though Mars somewhat more so. When mars is near opposition it is very conspicuously brighter than Saturn. When they are at similar magnitudes, they can be difficult to discriminate. Of course, you can always refer to an ephemeris and know for sure.