They are odd unless the clause is restrictive (no commas). Only the first makes temporal sense: The show being held this weekend features some 136 exhibitors. If you do not wish to be restrictive, it really needs a finite clause: The show , which is being held this weekend , features sme 136 exhibitors.
New words, one handy idiom, and a 2-minute quiz — delivered to your inbox to keep your streak alive.