"Either A, or B, or both"

Solution 1:

Typically you would list them in order with commas and leave off "or" until the last item, as in:

"either A, B or both are true"

Keep in mind that there are those who prefer a comma after B as well:

"either A, B, or both are true"

Solution 2:

Usually just "A or B" implies what you want, but if you want to clarify that meaning is not exclusive, you can use and/or.

Solution 3:

Mathematical logic speaks of an exclusive-OR and an inclusive-OR. Typically parents offer exclusive-OR choices to their children. ("You can have either cake or ice cream"; "...but not both" is implied.) You can say "Either a or b, but not both" to clarify an exclusive-OR, or "Either a or b, or both" to clarify an inclusive-OR. The "but not both" or "or both" in a sense wraps around the whole "either a or b" part.