I agree with all of the advice above. I personally have a different method of preparing feeders. I’m not saying mine is better or anything it’s just if you want to try something different. I place my frozen feeders in a plastic bag (not complete sealed) the I put it in the fridge for about 5-6 hours to thaw ( the time depends on the size of the pray I feed my young snake small mice). Then I heat up the mouse by putting it directly under the heat lamp for a few minutes. This always heats it up fast and works for me. Also three weeks really isn’t too big a deal. I remember when I first got my BP he didn’t eat for a month and I was super freaked out. Just last month the same BP just ended a four month feeding strike and he’s totally fine. Just weigh him every once and a while to make sure he’s not losing too much weight. But not for a while! Like said above let him settle in first and he’ll probably start eating.