Can't say as I have encountered this, if nothing has changed then surely there must be an inherent fault developed? Do the three engines rise in temps proportionally or is the main higher? Calculations ref increased surface areas and heat dissapation ought to be considered ref adding new plates methinks. We use a portable digital infrared Red Ray gun temp measuring tool for this type of situation, shine the two rays emitted onto the surface to be measured , move nearer or away to line up the two red dots shown and in the digital window is the temp of whatever you are shining it at. You can do it in C or F and store readings etc. good for either side of a Stat housing to see if he is fully open at the temp stamped on his body without taking the Stat housing off. An instant and accurate test. You can go all over the engine to measure heat exchangers in and out, ditto oil coolers, turbos, pto pumps, sea suctions / discharges , oil pipes etc etc, the list is endless. Good bit of kit costing around forty pounds over here. Also we use it a lot as some engine sensors display different resistance readings at different temps and to test them fully you need a cold start right thro' to max temp to be sure the particular sensor is operating correctly through the the total heat range and then compare all readings with manufacturers figures. You can also shine it on your workmates various body parts to ensure no localised overheating occurs ref overexcitment of whatever or if any imbalances are present, ie left v right?
Remembering The Good Old days, when Chiefs stood watches and all Torque settings were F.T.