First, though, I'd like to say "sometime" is not the word you're looking for. You need the s: "sometimes".
"Once in a while" and "from time to time" do imply something is more rare than "sometimes". But they generally mean the same thing. "From time to time" may also be more formal or "writerly" than "once in a while", if only because of some of the uses throughout history: "He shall from time to time give to the Congress information of the state of the union, and recommend to their consideration such measures as he shall judge necessary and expedient." is a sentence in the United States Constitution that references the "State of the Union" address: https://en.wikipedia.org/wiki/State_of_the_Union
The average speaker doesn't use "from time to time" very often. They can, and sometimes do, but it's more common to reach for "once in a while". "Sometimes" is more common still.