I can see someone pulling off taking more than three courses from their freshman to junior year. Senior year, especially the nonelective technical core courses though? If you're in your last year, I recommend taking only two over the summer especially if they're hard classes. For example, I'm a senior accounting major and decided to take only two upperdivision courses because it's less stressful/more manageable to earn A/B (thus keeping up my GPA - very competitive especially in my field, most firms won't even look at your resume unless you have a 3.0 or above. Trying to graduate with honors again and at a 3.5.) than having to take an additional similar/possibly likely more difficult course i.e. Federal Taxation. Speaking to a trusted academic advisor, professors, and peers helped me make the right decision.
It's really up to you, but personally if you honestly don't need to graduate soon or graduating is delayed by only like one semester, then the latter most likely will leave you a lot less stressed with more time to manage a life/school balance and really get the most out of learning from your courses. The thing I like about taking courses over the summer is that professors (based on my experience) cut down on the amount of chapters and eliminate group projects/papers they'd usually assign during other semesters
That's my position on taking summer courses. In the end, do what you think you can manage. If you think you can do well in taking a certain amount of courses as well as you wish to, then do so ^-^ If not, kind of better to play it safe and not take on too many hard courses at a time - increasing the potential risk of failing and having to take the course again, have your GPA negatively affected, and dealing with the pained expense of paying to take the course again.
Anyway, good luck