While I don't need your backrubs, I'll try to help you out on the topic at hand...
First, event-only approach... you MIGHT be able to do a very simple link to a common event in your random spell, which stores a randomized value from 0 to n (aka 0 to 2 in your case), which is followed by a conditional such as the following (written in fake syntax):
if (Random_Variable == 0)
cast Skill (002) # this is your fire spell
elsif (Random_Variable == 1)
cast Skill (003) # this is your water spell
elsif (Random_Variable == 2)
cast Skill (004) # this is your earth spell
end
As I said, this might or might not work, and I wouldn't seriously suggest it. But as I know the community, people like to take the easier-appearing solutions rather than getting a better one by trying to script it, so yeah... if this doesn't work and you can't get the scripted one done by yourself, post in Script Support/Request, and a funny fella might help ya out