Assalamu alaikum

Until recently, i was under the impression that the 3 rakah witr was part of the isha salah.. but this is not so. The witr salah is salah prayed AFTER Isha salah.

For Isha salah to be complete, you have to pray 4 rakah fardh. Although there are only 4 fardh, i would not say it is okey to miss the sunnah rakahs (4 before the fardh and 2 rakah after) as to be part of the Prophet's ummah you have to follow his sunnah.

Witr salah is waajib. Waajib is an act extremely close to fardh but not made compulsory like the fardh.

Ma'assalama