3 phase motors are indeed easy to connect but you must know what you are doing, there are a few different connection schemes to them.
recognizing the scheme takes a bit of learning and careful observation.
delta, double delta, wye, double wye, delta wye configurations, etc.
to name a few.
knowing how to diagnose a 3 phase system beyond basics is a skill learned over time.
training can be obtained for the senior maint. techs but its by far better to obtain a proper spark well versed in industrial to actually do the connections and testing, or at least oversee a proper training regime is the best recommendation!
proper isolation techniques is a valuable asset but for a maint. mechanic to isolate and test at the local disconnect (usually located at the machine) is about as far as they should go!
testing in a 3 phase panel without proper gear (ppe)and skills puts them in serious danger of arc flash injuries and should only be done by a competent spark who will have the proper ppe and knowledge to do so,