All 3mm fitted easily using a small socket and long set screw used together as a puller. Choose a tiny socket where the head of the set screw does not quite pull into the socket hexagon (3/16" socket) with the square drive end against the acrylic.... works like a dream. Except, for the holes for the Pi these seem to be (although I didn't measure them) to be slightly too small and the Microbarbs inserts did quite fit flush.
The Pi barbs are supposed to be 2.5mm. If you used 3mm they would be tight. Most people have said the 2.5mm barbs actually fit just a little loose.
Yes my mistake on the Microbarbs for the Pi. In this respect it would be handy to have packing lists for the bags of bits to indicate which are 3 and which 2.5 Anyone tried removing and refitting (in this case marginally smaller) Microbarbs? And it would be useful to put on the instructions here........ http://wiki.e3d-online.com/wiki/File:Feet_microbarb.jpg which bag the various sizes are in............as is done on the drawings to such good effect.