I recently attended a writers' conference. I met with three agents there, all of whom requested that I send in the first three chapters of my manuscript for consideration. Would it be best if I send my chapters to one agent and wait for a response before sending to another, or is it okay to send a partial to all three at once?
Send them all off at once. Agents who attend conferences expect multiple submissions. In fact, if you slink around in the Snarchives you'll find my rant on exclusivity (it stinks-don't do it).
You don't even have to mention how many agents are reading this unless asked. Multiple submissions is the default setting these days.
Your cover letter MUST mention in the first paragraph that the agent met you at Winnemuca Word Wrangle and asked to see your work.