I wouldn't ever do it, legal or no. One can't see the third vehicle in any MEANINGFUL way; the "crack the whip" effect is significant (dangerous), thus, emergency maneuvering ability is reduced to about nothing. I'm surprised that any state still allows it.
There are many reasons I stay far in front or behind commercial vehicles (problems observable, not just the usual safety factors), but any one pulling this rig is the type I either leave way behind or get off the road for a half-hour. Or, turn up the CB 'cause I'm waiting to hear about it from other drivers.
1990 35' Silver Streak Sterling
; 9k GVWR.
2004 DODGE Cummins 305/555; 6-manual; 9k GVWR.
Hensley Arrow. 10-cpm solo, 18-cpm towing
Sold: Silver Streak Model 3411