Y section tube

Is this what you want?
image


I made this using rhino6 for windows using cylinder and fillet edge
cylinder origin 0,0,0, diameter 1, length 2, direction X-axis.
with relocate gumball widget, origin 0,0,0, X-as direction enter, Y-as direction default.
rotate, duplicate with tab, angle 120 deg.
rotate, duplicate with tab again angle 120 deg.
select the 3 cylinder, boolean union,
fillet edge, radius 1, select the edges to fillet.
done

update: When you want to modify the Y-tube. See the explanation of Diego and Wim Is this normal? I think a bug. update: no bug but user error

Sorry for my bad english,it is not my native language