I'm not sure exactly what you're asking?? If you mean, how common is it to have three locomotives pulling a train, the answer would be that it's pretty common. If you're asking how often a long train will run with an engine up front, a helper at the rear, and another helper in the middle, I would say that happens but less frequently.
If you're asking something else, maybe you need to rephrase your question??
I was looking at a 44ton locomotive a few mins ago and I had a thought.
How many real or model railroads use 3 locomotives: first, middle, and rear? Was this as a common practice?
Mr. LMD, Owner, founder
The Central Chicago & Illinois Railroad