ok quickly here's how you do it:
create a particle system, add an attribute to it called index and make it a per particle float attribute.
Now create your objects to instance. E.g. a sphere, a cone and a cube.
select them and create a particle instance. In the instancer options there should be an option called "object index" change that to "index".
Now create the following expression in your particleShape "per particle (array) attributes" under the "index" entry (right click and create a creation expression)
particleShape1.index = rand(0, 2); (where particleShape1 is the name of your particleShape and the rand number is the number of objects you have to randomly choose from.
That's about it, and incidently this is roughly how i did the text effects in AVP earlier this year
see you learn something new everyday
Technical Director - Framestore
Currently working on: Your Highness