That is more like 3 branch lines departing from one supply pipe.. all the same diameter. The ratios of pipe flow area of each one to the total area of all branches will tend to set the flow into each branch, so they will want to be equal. Whether they will be equal or not depends on the downstream pressures of each branch.
If the flow to each branch is roughly equal, the velocity in the "header" will tend to reduce by 1/3 after passing the first branch and another 1/3 after the 2nd branch, but it will require that all downstream pressures near the header also remain roughly equal. The reduction of velocity at each branch, reduces friction in each succeeding section of the header, but that will not be significant, because the length of each segment is only a few meters.
As with all pipe flow analysis, you must know upstream and downstream pressures of each branch to determine the actual flow going into each one. If downstream pressures of the branches are different from one another, then flows could distribute unequally, otherwise flow area ratios will have the greatest effect.