If Monica took ten subjects, Marius took 10+4 = <<10+4=14>>14 subjects.
Marius and Monica have 14+10 = <<14+10=24>>24 subjects which they've taken.
Since Millie is taking three more subjects than Marius, she is taking 14+3 = <<14+3=17>>17 subjects
Together, the three have taken 24+17 = <<24+17=41>>41 subjects.
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