What 3 classes are they? I'm assuming that includes anatomy and physiology? Do you have to take the lab portion(s) as well?
Can you split the 3 classes up over the summer like some schools have summer session 1 and a summer session 2?
What about trying to take one of them online? Will your school allow that?
I'm not sure what your specific requirements are. But my advice is if you have the time to do it--do it!
It's possible, I took 4 classes last summer and got all A's.
But this is assuming you don't have to work during the summer, or have something else eating up your time.
My first few years of undergrad I made the mistake of not using my time efficiently. I always thought to myself 'Oh, I can take that later' or 'that class can wait until next year', etc. In retrospect I wish I had taken my Anatomy, Micro, and Physiology earlier during the school year, instead of waiting, because I would have been able to apply earlier to more nursing programs