It's perfectly normal to want some closure, so don't think you're stepping on anyone's toes. Your plans are important too! Something like this would work:

"Hey there,

I interviewed with B three weeks ago and was told to expect a response within 1-2 weeks. I'm still interested in working for XX, and I'm wondering if you were also interested in moving forward to any subsequent interviewing steps.

(close with a nice pleasantry, thank them for their time)"

Be polite, brief, and straightforward, and you're more likely to get an answer.