How to calculate the cumulative GPA from courses from 3 high schools?

My kid is trying to calculate her cumulative HS GPA. However, she has grades from three different schools and we’re unsure how to do this. She needs a cumulative GPA (freshman, sophomore, and junior years) for college applications. Should she leave out grades like physical education and art and include only the core academic courses such as English and science? Should she weight the honors courses at all? What about a remedial math she decided to take while she also took a Geometry course? She also had - and + grades, but the calculators she found online don’t include minus and plus grades. Any advice? Thanks!

What process does her current school use for GPA? She should use that for calculating/weighting. Are they not considering her grades at the first two schools?

This calculator has pluses/minuses. You can ask her current guidance counselor to help you determine what courses to use in the calculation.
http://gpacalculator.net/college-gpa-calculator/

Thank you!