Methyltrienolone is only really useful as a pre-workout. Like test or tren base, you're not going to see a lot of growth on it due to the short half-life.
But it is extremely liver toxic. Don't think it's something you can "flush out", and running it for three weeks is crazy. It's not dbol. It also doesn't make a lot of sense to kick start a cycle with an atom bomb

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I use .2ml of a test base 50mg, tren base 50mg & methyltrien 5mg blend once a week prior to squats. And run NAC and Tudca along with it. Despite that, it spikes my liver values. Not alarmingly, but enough to realise plenty of caution should be applied.
If you want to gain mass and use exotic substances, perhaps consider trestolone/MENT.