I personally run two regular bottles for IM…one BTA and one on my downtube (usually a large insulated CamelBak). At Special Needs, I have another insulated CamelBack to pick up.
I start the race with the Camelbaks frozen…small insulated cooler in my Special Needs bag keeps it cold enough so it is still cold when I get to it. A small cooler bag does the trick and fits in a Special Needs bag. Both those bottles are calories and water goes BTA. I swap out water as needed at aid stations. I’ll also supplement calories by grabbing on-course nutrition, taking a big swig and dropping it before the end of the zone.
You really don’t need more than that on the bike…as mentioned there are ample aid stations on the course and, as long as you don’t need special nutrition, is more than sufficient to fuel and hydrate properly.