How can I randomize some specific blocks order?

[Brenda Salenave Santana]

Hello, 

I am trying to construct a form where the first three blocks order are always the same and the rest should be randomized. However, consulting the randomize guide [https://github.com/rubenarslan/formr.org/wiki/How-to-randomize-items-and-blocks] I couldn't figure out how to do this randomization just for some of the blocks.