What's needed to operate MTH trains?

The answer isn't entirely simple. Conventional mode is suitable for all, but some features aren't available.  The 1 gauge, some O gauge and HO should operate under DCC if you have that and sufficient amperage.  If the O gauge are PS1, you've got all the features you are going to get with conventional AC (no DCC, no command). If the O gauge are PS2 and PS3 they may have DCC, but they all should respond to DCS command through a TIU, a wi-fi TIU (more coming apparently) or the limited commands of the DCS Explorer.  So check the manuals, if you have them, or consult a knowledgeable dealer or friend as to what you've got in terms of what they will operate with.  If the manual doesn't mention DCS or DCC, you've got something that will work only with conventional AC (O gauge) or DC current (HO, large scale.).

They are all new 3.0. What equipment do I need to purchase.  Thanks,  Dave

For PS3.0 you need at least one of the following:  DCS Explorer (limited to 3 locos and not all higher level functions) (not currently in production to my knowledge); TIU and DCS handheld remote (not currently in production and no production planned for the future to my knowledge); DCS wi-fi TIU + DCS app (several versions) + smart device (phone or tablet).  This last is planned for production according to recent reports but not currently available.  Indeed, a major problem for new MTH owners at present is that almost all the DCS market is used devices at much higher than previous prices, with no warranty.  If you're patient, this is likely to change through availability of the new wi-fi TIU.  Your best bet is to locate an experienced and reliable dealer who can keep you informed.  If there is no one local, consider some long time and well known MTH dealers such Mr. Muffin, Stockyard Express, Ready to Roll, etc.