It is very hard to make out details even when only two volumes are blended, therefore there is no option on the GUI to blend more volumes.
You can easily combine any number of volumes, especially if they have the same geometry (image extents, origin, spacing, axis directions), using numpy. See this example in the script repository: https://www.slicer.org/wiki/Documentation/Nightly/ScriptRepository#Combine_multiple_volumes_into_one