This is the simplest solution:
First assemble 3 piece with 2 notches to build a cradle then add the 3
pieces with one notch each standing up to hold this fragile
constellation together. Note, if you do not have any pieces with just one
notch then use pieces with 2 notches instead.
Now take three more pieces with 2 notches and add them as shown as a
ring around the other six pieces. They can only be moved in as a unison.
Last add the three pieces with 3 notches each. Again they can only
slide in as a unison. But it helps to put one in first, then the next
one, then take out those two again, add the third one and slide all
of them back in.
There is another solution if you start out using 2 pieces with 2 notches
and 1 piece with 3 notches to build the lowest cradle. It ends with
sliding in a few pieces at the end together, but is basically the
same solution as the one above.
If your puzzle has only pieces with 2 or 3 notches (none with just 1) then there is a third solution where the pieces with 3 notches are parallel.
I personally prefer puzzles where there are three pieces with only 1
notch because that leaves no internal holes within the hexagonal wooden
pieces. There obviously is still a central hole in the shape of a
rhombic dodecahedron.