Engineering - It's a matter of some pride here at Core Corp that we've never under-engineered anything.* That isn't to say that all products must be over-engineered, of course. Merely that it's better to be safe than sorry. In regards to the inclusion of a third engine, there are a number of reasons for this: propulsive symmetry, Bussard coverage, and acceleration. You can't prove that these ships won't be used in evasion.
*Almost certainly untrue, but Core Corp has been around for a very long time. Even corporation historians can't agree on Core Corp's product lineup of the past hundred years, to say nothing of the thousand years before that.
Business Development - Talk of symmetry doesn't answer for the cost of an additional engine. And I actually pulled up the latest drawings this morning (I'm in the repository, in case you've forgotten), and I couldn't help but notice that your people have spec'd Type R engines for this thing instead of the standard Type N? Who are you trying to outrun?? The engineering team in charge of the Pursuant corvettes are only using ONE Type R and advertising it as the fastest tank-classed prototype in history! Why would a low-level planetary transport that masses barely twice as much need THREE Type R's?
Engineering - Planetary piracy is a thing that happens. Making the ship viable in private sector space as well as public sector space requires the ability to avoid looting at all costs.
Design - I'll pipe in and point out that the tri-engine design is fairly heavily patented by The Chef Boy Ardee Conglomerate, esp. in regards to placing landing strut enclosures in the bottom two engines. FYI - The lateral twin-engine design patent is held by yours truly*, and it's a beautiful match of form and function if I do say so myself.
*The patent for "A Lateral Twin-Engine Post Atmospheric Layout" patent was co-filed by Daymy Sherrin and Kilmoot Drastisist on behalf of Core Corp. It differs from most lateral twin-engine designs based on the inclusion of vestigial wings.
Business Development - "Avoid looting at all costs?" I can promise that nobody wants the cost of this thing just to avoid the remote possibility of looting. Get rid of the third engine and go with whatever that lateral-twin thing is. And make the engines Type N, there is NO market for a planetary transport packing the stats and PRICE those engines bring with them, and we are NOT eating the margins on them for the sake of whatever misplace pride your engineering team thinks they have. If nothing else will convince you, the patent thing from CBAC is sorta non-negotiable. Mainly because A) we will NOT be eating a licensing fee on this ship, and B) I won't negotiate it.
Engineering - We're not violating CBAC's patent. Daymy, you're either looking at old drawings or you've got your screen upside down. The twin engines are on the top.
Design - Oh lordy... I'm not styling this thing. You folks are on your own.