High leverage is only beneficial for winning trades. So its best to apply them on trades with higher potentials.
If you use higher leverage and trade goes in an opposite direction as expected then it can make losses. You should choose an optimal leverage ratio as it increases the risk factor on your account balance. I would suggest you to go for 100x or 200x leverage if you are not so experienced. If you have years of experience, you can choose more based on your risk management.
I'd almost never touch non index 3x ETF, especially a cyclical race to the bottom industry. Just look at the famous FAS. Look at what they've done to our boy.