What are your opinions on having two 3 hour labs in one day? Do you think this is too much? Has anyone done this before?
I know people who have done it and best to avoid if possible. Yes, MUEP chem does it, but that's different because they don't have things due a week later (which your labs may or may not have - don't know what year you are

), but they're also not as regular. Very big difference between having 2 labs back to back once a month to having it once a week.
If it's to avoid a commute, recommendation: skip your lecture, take that time to rest. Also, what units is this for? Because if it's, say, BIO1022 and EAE1022, welp the first only has labs every second week, and the second usually has really chill labs that you can come and go from as necessary, so that would be fine. But if it's something like PHS1022 and CHM1022, avoid, do not do, back-to-back is a horrible idea.