You may not be seeing it, the rocket three dimensionally as a stack.
Try this. Maybe it will help my explanation if you see an R-7 function.
I'm still unsure of your point. Yes, later developments of the R-7 had an upper stage (and even several) after the core burned out. The original, and the Semyorka which launched Sputnik 1, did not.
I'd be grateful if you explained again.
I would point out, that the Russians (who are no slouches when it comes to rockets) use pairs and fours with their strap-ons. The Americans think they can wrap 5-7-9 candles around a Delta and get away with it, but even they do not use the 3 strap-on configuration. It's actually too dangerous if one of the strapons fails or burns un-even.
Fair, but it's the same with any double booster configuration as well (such as, say, Titan). An uneven burn is always incredibly dangerous to a launches chance of success. I would argue that a tri-booster set up may be
more dangerous, but not that it is
too dangerous.
It does make me wonder about switching to four for a crewed booster though...