There's a similar question right now, and I have an issue: How would one write the grade of A in plural? For clarity, my smart device isn't smart enough for italics. I will use bold.
In general, I use The Chicago Manual of Style. There is no problem with "I got three Bs and a C." (No italics recommended.) However, to say "I got straight As" is a problem. Or maybe it's not a problem, but I just don't remember ever seeing it. To combine roman and italic in the same line is also a problem: "I got two A s, two Bs, and an F." I'm tempted to use an apostrophe, but I don't like that option either: "I got two A's and two Bs."