High Speed Ducer

Can I use the Mega DI ducer for my high speed depth reading?

splitting the MSI and MDI with the 9M SIDB y cable?

What display do you have? It sounds like you may have a Mega SI tranducer and if so you can run split screens or individual screens on your display to display side imaging, down imaging and 2D imaging at the same time without a splitter cable. You may even be able to view all three at the same time on the same screen.
Using your Mega SI transducer may not be the most favorable option for a high speed display reading but it can be used for lower speeds.
Your best option may be to get a high speed transducer and a splitter cable and run signals from both transducers into your display at the same time. Get the transducers set up properly in your display and then it’s just a matter of setting up your views(SI, DI or Dd) how you want to display them.
I don’t know how everyone prefers to run their SI, DI and 2D imaging but on my Helix 12 I run both transducers using a splitter cable and it works very, very well.
The side imaging transducer is a fairly pricey transducer too and running two transducers will allow you to raise your side imaging transducer slightly above the bottom of the hull to minimize any damage that may occur to your side imaging transducer should you happen to hit bottom or an object.
There’s also a ton of info on the Humminbird website and other websites that may give a much better answer than I can.

Maybe I should be more clear.

here is what I have.
Helix 12 mega SI/DI Gen 2
Helix 10 Mega DI Gen 3
Helix 7 all season. gen 3

Putting it on Ranger 1880ms

I am going with externally mounted transducer(s)

which transducers should I mount and do I need a splitter cable? I have 3 transducers Mega SI, Mega DI, and standard chirp(came with Helix 7).

Any or all of them network capable? If network capable, you only would need the msi transducer to have it all on any display. You will need a network hub for all those units.

All are Networkable. I thought the MSI ducer doesn’t read well at high speeds?

Whee are you planning on mounting your displays? Two on dash one on the bow and if so, which ones where?
If running one in the bow, will you use the transducer on your trolling motor?

10 and 12 on the dash. 7 on the bow will run on the TM ducer.

I agree with Karry above.
You will need your MSI transducer, high speed transducer with a spliiter, network hub and adapter.
Depending on your trolling motor setup will determine whether you want to also pull from that transducer to your network as well. Your 7 gn3 isn’t Mega so that limits that units read ability.