If English makes 3 additional gender terms to accommodate for XXX, XXY, and XYY people, what would be the most realistic terms for those genders?

Most babies aren't screened for their chromosome number. I have always assumed I'm XY, but I don't actually know, and I've never felt that it was urgent to check it. XXX and XYY individuals usually look outwardly like normal females and males, respectively. Geneticists use the terms "superfemale" and "supermale," but outside of technical language we probably should not use those words. XXX females do not necessarily look like Supergirl or Wonder Woman or Barbarella; they tend to be tall and may have (but don't always have) minor developmental delays, but most lead quite normal lives. XYY males do not necessarily resemble Superman or Jason Momoa; they also tend to be tall and sometimes show developmental delays, but not always. Many XXX and XYY individuals are never diagnosed. Since you can't pick XXX females or XYY males out of a crowd just by looking, there's probably no need for a special English term.

Most importantly: you're mixing up sex and gender. Sex refers to what the good professor Ken Saladin calls the "three Gs" -- genes, gonads, and genitals (which usually, but not always, align with one of the two typical patterns). Gender refers to what a person knows themself to be: according to Merriam-Webster, it is "the behavioral, cultural, or psychological traits typically associated with one sex." There is a considerable and growing amount of evidence that gender is encoded in brain structures and can't be changed consciously. Usually, the "three G's" align with gender in typical ways: a kid with X and Y chromosomes, testes, and a penis will usually self-identify as a boy and do things that our culture says are "boy things." Usually. Not always.

XXY males have male genitals. The testes are small, and they're infertile, but I see no reason to call them anything other than "males." If you have to refer to their chromosomes specifically, you can say "Klinefelter's syndrome." But again, if you can't pick them out in a crowd without detailed physical examination, there's probably no need for a special English term.

I am still studying engineering. I feel worried being an average student. Can I get a good job in placement, buy a house, and a car? I don't know why I feel this.

So the karyotypes XXX, XXY, and XYY aren't genders at all, and in non-technical English, there's no real need for specific words for them.

You left out X0 (Turner's syndrome), XXXX, XXYY, XX with translocated SRY, XY with deleted or damaged SRY, XX with partial or complete androgen insensitivity, güevedoce. . . .