This is actually not correct.
below is right for the DOCs
"When an Engineer unit has less than 100 men, the probability of completing the bridge is reduced proportionally. In addition, an engineer company will have a probability of bridge completion 1/3 that of an engineer battalion. Otherwise, having more than one Engineer unit constructing a bridge will increase the chances of completion."
So you it is up to you is you want ONE chance at the normal rate or 3 chances at 1/3 the normal rate.
Also note there is a Quality Modifier so better Eng take longer to build then poorer engs.
FWIW Guys - we did a lot of work during PzC Bulge 44 on how long it takes to build bridges vs the ENg unit size and quality. So while it seems like the Bridge takes forever, statistically for the Germans crossing the Our RIver at thr start of thr Bulge Campaign, your bridges will generally be completed more quickly than they were built historically.
Thanks Glenn. However it is, it seemed to me that my chances were greatly increased, and it sure seemed to go smoother, when I built bridges with companies. But mathematically, I think it actually is better to take 3 chances at anything rather than 1. jonny