just read the bit about the pipes and their diameters, in an air conveying system (although this is smoke it's still air being used to convey it), the action of reducing the diameters is to increase the pressure as it moves from the nearest offshoot to the furthest, all the outlet diameters and the pipes for each run should be the same, i.e. each branch should be the same, that means for 3 connections they should all be the same irrespective of their lengths, so the furthest is, say, 100mm dia then the second furthest should be 100mm as should be the third, the diameter change from the furthest to the second furthest should be as close to the branch where they meet, doing a trouser branch (60° equally branch) is ideal, the diameter back should be equal to the same cross sectional area of the two connections, the same applies for the connection to the closest connection though here the problem is that the small diameter and the furthest branches run of pipe diameter is different so a 30° branch is used with a cone down to the larger diameter from the feed duct, again is now sized at the same cross sectional area of all three connections (4 connections at 100mm would equate to a doubling in the diameter of a duct to 200mm).
this ensures that the pressure is the same across all 3 ducts, as air will like water take the least line of resistance, in practice calculations are taken based upon the furthest duct when sizing a fan for the pressure and duty, at this scale however, all the above is throw out of the window as the differing diameters, lengths and fan pressure will be to small to make a noticeable difference
, also in practice, dampers are used to throttle the air pressures, where a damper could be almost 2/3's closed before it had any effect and then the effect would be quite big or little depending upon how the system wanted to behave.
You could also have 3 lines of piping all identical in length and that would create the ideal equal pressure system.
As you discovered, small cones are not as efficient in airflow as longer cones, the back pressure is more prenounced so the speed reduces.