Methadone is indeed used succesfully to taper off of heroin/other opioids. I was on methadone maintenence at 75mg per day for about a year and a half; when they say that methadone withdrawals are worse than heroin withdrawals, this is indeed true, but they mean from large, long habits. I detoxed cold turkey from 75mg per day for 1.5 years and it was the most intense detox I have ever gone thru, including from a 2 year IV heroin habit. Th eproblem is, the half life is soo long that the withdrawals last forever, sometimes more than a month..
BUT..
The good news is, you don't have to worry about that. Using 3 10mg pills won't be enough to cause you to experience any withdrawals from the methadone. Your habit wasn't very big luckily, and 1 month is not a very long time. Certainly I believe you about your withdrawal symptoms, but in the grand scheme it shouldn't be ridiculously bad.
You can certainly use the 30mg of methadone you have to succesfully detox from heroin. You want to do a short taper, ideally around 5 days. Methadone is quite potent, you shouldn't need much to hold a moderate heroin habit. The 10mg pills you have are the strongest ones, as the pills only come in 10 and 5mg. If I were you I would do the following:
Day 1: 10mg
Day 2: 7.5mg
Day 3: 5mg
Day 4: 5mg
Day 5: 2.5mg
Day 6: Stop
This way you are decreasing your dose as you go, and won't be taking enough to get dependent on the methadone. 5 days should be enough to get the heroin detox overwith, and while you may not feel like a million bucks, using th emethadone in this fashion should certainly make you feel okay enough to move around, possibly go to work, whatever. Once you stop after the five days, you may still feel some minor residual withdrawal symptoms, but nothing you can't handle. Probably soem minor restlessness, anxiety/depression, etc.
The hardest thing about getting clean usually isn't th einitial acute detox. It is the post acute withdrawal syndrome, that comes into play after the physical symptoms dissipate. This means the cravings, anxiety, depression, insomnia, etc. It is important to have a solid plan in order on how you are staying clean. Whether it be some kind of rehab, going to meetings, seeing a therapist, etc. Because if you just get sober and dont' change anything in your life, you are bound to relapse.
Coming off of a 1 month heroin binge should not be too bad though, so I am confident you will make it, just stay strong and don't get discouraged. I wish you luck, and if you need any help with opiate withdrawal feel free to private message me as I've been dealing with it for quite some time and am pretty experienced when it comes to that.
Mike