Short answer:
No Lionel's system can't control DCS locomotives without a TIU because the 2 systems communicate differently.
Long answer:
TMCC/Legacy use one way radio communication. If you tell locomotive #2 to blow its horn the TMCC/Legacy base sends out the command "locomotive #2 blow your horn". Locomotive #2 sees the command and blows its horn. If it misses the command the TMCC/Legacy base has no way of knowing.
DCS is two way communication through the positive and negative rails. When you tell locomotive #2 to blow its horn the TIU sends the command out through the rails, the locomotive receives the command, it responds back to the TIU it got the command, and blows its horn.
The two systems operate differently, so without MTH electronics inside the Lionel Base 3 there's no way it's controlling a DCS locomotive. MTH has no incentive to do this, as they wouldn't sell any TIUs themselves.
A TIU can control a TMCC/Legacy locomotive if you hook the TIU to a Lionel TMCC/Legacy base using the proper MTH cable. The serial port on the Lionel base is designed to let a computer or something else input commands to the base instead of a Cab1, Cab1L, Cab2, etc. So basically the TIU is just sending commands to the Lionel base instead of the Lionel remote. I have all my PS2, PS3, TMCC, and Legacy locomotives programmed into my DCS remotes. I can run any of my locomotives from a DCS remote with my TIU connected to my TMCC base.
So long of the short is if you want to be able to operate a DCS locomotive from a Lionel Base3 Lionel and MTH would have to get together and develop a way for the Base3 to talk to a TIU and tell it what commands to issue to DCS locomotives.
