What are your goals? I think that might help to get the best answers.
Did you want to be able to use the Digitakt to sequence the 3 synths, or just use one synth at a time with the DT? Your post is a bit confusing because it says you want to hook them all up, but also that you only use one at a time because of space. Are you looking to use the C8x as a midi keyboard for the DT, or just use the keys on the device?
A couple thoughts below. Beware, I’m making a bunch of assumptions.
If you want to hook everything up at once and sequence/clock from the DT, you will need a midi-thru box since (as far as I can tell) the Cobalt and Shruthi don’t have thru. You could go DT out > Boog In, Boog thru > one of the other two synths (but then you’re blocked)
I’m assuming that you want to run the boog through either the DT or C8x to take advantage of the internal fx? I’m not sure if the Cobalts line-in is pre or post filter, but that might be worth considering.
If you want an easy way to add inputs without breaking the bank, there are some small powered mixers out there for cheap. I bought a Moukey MAMX3 to consolidate a few mono synths that I don’t need on exclusive channels, and I’m astonished at how good it is for the price (it’s still pretty new so I can’t comment on long term durability). Its also really small, so I just Dual-Locked it to the bottom of my desk, plugged shit in, set levels, and haven’t thought about it since. Something like it might be good for you if you’re setting up/breaking down every time