Any vector →V can be written in terms of three non-coplanar vectors
hence,
→V=p(→a×→b)+q(→b×→c)+r(→c×→a)
where (→a×→b),(→b×→c),(→c×→a) are non-coplanar vectors and p,q,r are scalars.
now, taking dot with →a:
we have,
→V⋅→a=q[→b →c →a]=0⇒q=0,
similarly,
→V⋅→b=r[→b →c →a]=0⇒r=0
and
→V⋅→c=p[→a →b →c]=0⇒p=0
So, →V=→0
⇒|→V|=0