How to use variables while calling pybedtools
You have to take your variables and make them into a single string variable.

for example:

chr_ = "chr1"
start = 3000
end = 3402
as_str = ' '.join([chr_, str(start), str(end)])
# or 
as_str2 = "{} {} {}".format(chr_, start, end)


Once you have the string built you can use the example in the bedTool docs.

a = BedTool(as_str, from_string=True)